As noted last week, find_removable_extensions initializes several 
bitmaps, but doesn't clear them.

This is not strictly a leak as the GC system should find dead data, but 
it's better to go ahead and clear the bitmaps.  That releases the 
elements back to the cache and presumably makes things easier for the GC 
system as well.

Bootstrapped and regression tested on x86_64-linux-gnu.

OK for the trunk?

Jeff