treewide: remove unnecessary includes from header files

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>