once we start offering 1.5.0.x users major updates to 2.0.0.x or 3.0.0.x, some users might want to go back to 1.5.0.x because they want to (or because they have to.)
this fix tries to do a "best effort" job of removing any of the Fx 2.x / 3.x files that upgrading could have given them that would interfere when going back to Fx 1.5.x with a "pave over" install.
r=rstrong, sr=dveditz, a=dveditz
git-svn-id: svn://10.0.0.236/branches/MOZILLA_1_8_0_BRANCH@220129 18797224-902f-48f8-a5cc-f745e15eee43
85a55c0bcf