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>

Documentation/CodingGuidelines

@@ -444,7 +444,7 @@ For C programs:
  - The first #include in C files, except in platform specific compat/
    implementations and sha1dc/, must be either "git-compat-util.h" or
    one of the approved headers that includes it first for you.  (The
-   approved headers currently include "cache.h", "builtin.h",
+   approved headers currently include "builtin.h",
    "t/helper/test-tool.h", "xdiff/xinclude.h", or
    "reftable/system.h").  You do not have to include more than one of
    these.