tree compose: Delete .dbenv.lock and __db.* files from /usr/share/rpm

Currently on an Atomic compose, I'm seeing abrtd trying to write to
/usr/share/rpm/.dbenv.lock, which is denied by policy because it's
usr_t.  There are multiple ways to address this, but there's no good
reason to leave the lock files and __db* files around.

rpm appears to operate correctly without them if calling process
merely gets EROFS.