On the test workload (mpv --cache=yes with VP9 video decoding), the
bin scanning has a very poor success rate (less than 2%). The tcache
scanning has about 50% success rate, so keep that.
Update comments in malloc/tst-memalign-2 to indicate the purpose
of the tests. Even with the scanning removed, the additional
merging opportunities since commit 542b110585
("malloc: Enable merging of remainders in memalign (bug 30723)")
are sufficient to pass the existing large bins test.
Remove leftover variables from _int_free from refactoring in the
same commit.
Reviewed-by: DJ Delorie <dj@redhat.com>