Come disinstallo manualmente gli Strumenti per gli sviluppatori?

Sì, esegui semplicemente quanto segue in Terminale:

sudo <Xcode>/Library/uninstall-devtools --mode=all

Dove <Xcode> è l'installazione predefinita di Xcode, in genere:

sudo /Developer/Library/uninstall-devtools --mode=all

In base al tuo OP, presumo che tu stia utilizzando Xcode pre-Lion. Questo è spiegato nella documentazione fornita con l'installazione di Strumenti per sviluppatori:
/Developer/About Xcode

Uninstalling Xcode Developer Tools
To uninstall Xcode developer tools on the boot volume along with the directory, from a Terminal window type:
$ sudo /Library/uninstall-devtools --mode=all

To remove the underlying developer content on the boot volume, but leave the directory and supporting files untouched, from a Terminal window type: $ sudo /Library/uninstall-devtools -- mode=systemsupport To just remove the UNIX development support on the boot volume, but leave the directory and supporting files untouched, from a Terminal window type:
$ sudo /Library/uninstall-devtools --mode=unixdev

Finally, to just uninstall the directory you can simply drag it to the trash, or from a Terminal window type:
$ sudo /Library/uninstall-devtools --mode=xcodedir

NOTE: The uninstaller that ships with previous versions of the Xcode developer tools will not clean everything off of your system properly. You should use the one installed with these Xcode developer tools.

Controlla super-doppio controlla che questa documentazione esista o meno con la tua installazione o sia stessa come tua installazione prima di agire. Come con qualsiasi di questi comandi, la precisione e la compatibilità sono critiche .

Sembra che devi solo rimuovere la cartella /Developer (specialmente se hai già installato la versione 4.3 o successiva di Xcode). La sezione Xcode del Centro assistenza sviluppatore Apple ti dice in realtà di eseguire questa operazione (sottolineatura mia).

I've installed the latest version of Xcode from the Mac App Store, but a previous version of Xcode (4.2.1 or earlier) keeps launching. What should I do?

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To run the latest version of Xcode, make sure you are launching Xcode.app from the Applications folder. Xcode 4.3 and later is packaged on the Mac App Store as a single application and installs in your Applications folder as Xcode.app.

Previous versions of Xcode placed the tools and frameworks in the Developer folder. You can delete the older version of Xcode by dragging the Developer folder to the trash.

Questo è reiterato in Nuove funzionalità in Xcode 4.3 (sottolineatura mia).

The simplification afforded by repackaging Xcode 4.3 as a single app bundle eliminates the need for the /Developer directory containing prior versions of Xcode. As a result, the Install Xcode application and the uninstall-devtools command line script are also no longer needed.

Note: The first time you run Xcode 4.3, you are prompted with a dialog to delete the Install Xcode application from the Applications directory if one is resident, and to remove any older installation of Xcode (Figure 4). These actions are optional, Xcode 4.3 can co-exist with older installations of Xcode on the same Mac. Some operations may require an administrator password to enable them if you switch from one version of Xcode to another.

Figure 4 Option to remove older versions of Xcode

If you choose not to delete these items on the first launch of Xcode 4.3, you can remove the /Developer folder and the Install Xcode application by dragging them to the Trash at any time.