cache.h: remove this no-longer-used header

Since this header showed up in some places besides just #include
statements, update/clean-up/remove those other places as well.

Note that compat/fsmonitor/fsm-path-utils-darwin.c previously got
away with violating the rule that all files must start with an include
of git-compat-util.h (or a short-list of alternate headers that happen
to include it first).  This change exposed the violation and caused it
to stop building correctly; fix it by having it include
git-compat-util.h first, as per policy.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -72,10 +72,6 @@ struct strbuf {
 extern char strbuf_slopbuf[];
 #define STRBUF_INIT  { .buf = strbuf_slopbuf }
 
-/*
- * Predeclare this here, since cache.h includes this file before it defines the
- * struct.
- */
 struct object_id;
 
 /**