wrapper: reduce scope of remove_or_warn()

wrapper: reduce scope of remove_or_warn()

remove_or_warn() is only used by entry.c and apply.c, but it is
currently declared and defined in wrapper.{h,c}, so it has a scope much
greater than it needs. This needlessly large scope also causes wrapper.c
to need to include object.h, when this file is largely unconcerned with
Git objects.

Move remove_or_warn() to entry.{h,c}. The file apply.c still has access
to it, since it already includes entry.h for another reason.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>