treewide: remove unnecessary cache.h includes

We had several header files include cache.h unnecessarily.  Remove
those.  These have all been verified via both ensuring that
    gcc -E $HEADER | grep '"cache.h"'
found no hits and that
    cat >temp.c <<EOF &&
    #include "git-compat-util.h"
    #include "$HEADER"
    int main() {}
    EOF
    gcc -c temp.c
successfully compiles without warnings.

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

sequencer.h

@@ -1,11 +1,11 @@
 #ifndef SEQUENCER_H
 #define SEQUENCER_H
 
-#include "cache.h"
 #include "strbuf.h"
 #include "wt-status.h"
 
 struct commit;
+struct index_state;
 struct repository;
 
 const char *git_path_commit_editmsg(void);