treewide: remove unnecessary includes from header files

There are three kinds of unnecessary includes:
  * includes which aren't directly needed, but which include some other
    forgotten include
  * includes which could be replaced by a simple forward declaration of
    some structs
  * includes which aren't needed at all

Remove the third kind of include.  Subsequent commits (and a subsequent
series) will work on removing some of the other kinds of includes.

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

submodule-config.h

@@ -2,7 +2,6 @@
 #define SUBMODULE_CONFIG_CACHE_H
 
 #include "config.h"
-#include "hashmap.h"
 #include "submodule.h"
 #include "strbuf.h"
 #include "tree-walk.h"