Remove unneeded header

Signed-off-by: NUwe Zeisberger <zeisberg@informatik.uni-freiburg.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

mailsplit.c

@@ -11,7 +11,6 @@
 #include <sys/stat.h>
 #include <string.h>
 #include <stdio.h>
-#include <assert.h>
 #include "cache.h"
 
 static const char git_mailsplit_usage[] =