Delete check-racy.c

Delete check-racy.c

This is git-checy-racy command, added a long time ago [1] and was never
part of the default build. Naturally after some makefile changes [2],
git-check-racy was no longer recognized as a build target. Even if it
compiles to day, it will not link after the introduction of
common-main.c [3].

Racy index has not been a problem for a long time [4].  It's time to let
this code go. I briefly consider if check-racy should be part of
test-tool.  But I don't think it's worth the effort.

[1] 42f774063d (Add check program "git-check-racy" - 2006-08-15)
[2] c373991375 (Makefile: list generated object files in OBJECTS -
    2010-01-26)
[3] 3f2e2297b9 (add an extra level of indirection to main() -
    2016-07-01)
[4] I pretend I don't remember anything about the recent split-index's
    racy problem

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>