oidset: remove unnecessary include

When commit 8b2f8cbcb1 ("oidset: use khash", 2018-10-04) moved from
using oidmap to khash, it replaced the oidmap.h include with both one
for hashmap.h and khash.h.  Since the hashmap.h header is unnecessary,
and the point of the patch was to switch from hashmap (used by oidmap)
to khash.h, remove the unneccessary include.

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

oidset.h

@@ -1,7 +1,6 @@
 #ifndef OIDSET_H
 #define OIDSET_H
 
-#include "hashmap.h"
 #include "khash.h"
 
 /**