Revert "test_cmp: diagnose incorrect arguments"

Revert "test_cmp: diagnose incorrect arguments"

This reverts commit d572f52a64c6a69990f72ad6a09504b9b615d2e4; the
idea to detect that "test_cmp expect actual" was fed a misspelt
filename meant well, but when the version of Git tested exhibits a
bug, the reason why these two files do not match may be because one
of them did not get created as expected, in which case missing file
is not a sign of misspelt filename but is a genuine test failure.

Acked-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>