From this question I known how to remove vendor installed apps.
However is it safe to uninstall Flash and Facebook? I guess that the Android browser depends somehow on Flash and might brake after I remove Flash package.
I simply want to have more space for other apps and don't want to be reminded every time about updates for Flash and Facebook.
Update: From my point of view both answers are correct. In general it is not safe to uninstall vendor installed apps. However my HTC Desire won't have any new OTA updates so in my case is it safe to delete them.
I just found out that latest ROM update (with Android 2.3) for HTC Desire isn't available as OTA update, you have to install it manually. And Facebook app is already removed from the ROM.
Unfortunately I cannot mark both answers as correct therefore I choose the answer with higher number of votes as correct one.