Revert "check_packed_git_idx(): check integrity of the idx file itself."

This reverts c5ced64578a82b9d172aceb2f67c6fb9e639f6d9 commit.
It turns out that doing this check every time we map the idx file
is quite expensive.  A corrupt idx file is caught by git-fsck-objects,
so this check is not strictly necessary.

In one unscientific test, 0.99.9m spent 10 seconds usertime for
the same task 1.1.3 takes 37 seconds usertime.  Reverting this gives
us the performance of 0.99.9 back.