bugreport: drop extraneous includes

In the generic parts of the source files, system headers like <time.h> and <stdio.h> are supposed to be included indirectly by including "git-compat-util.h", which manages portability issues. Drop our explicit inclusions and rely on "cache.h", which includes "git-compat-util.h". Signed-off-by: Emily Shaffer <emilyshaffer@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>