packfile.h: drop extern from function declaration

In 336226c259 (packfile.h: drop extern from function declarations,
2019-04-05), `extern` was removed from function declarations because
it's redundant. However, in 8434e85d5f (repack: refactor pack deletion
for future use, 2019-06-10), an `extern` was mistakenly included.

Remove this spurious `extern`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

packfile.h

@@ -100,7 +100,7 @@ struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
  * Does not unlink if 'force_delete' is false and the pack-file is
  * marked as ".keep".
  */
-extern void unlink_pack_path(const char *pack_name, int force_delete);
+void unlink_pack_path(const char *pack_name, int force_delete);
 
 /*
  * Make sure that a pointer access into an mmap'd index file is within bounds,