tests: index: do not re-allocate index

Plug a memory leak caused by re-allocating a `git_index`
structure which has already been allocated by the test suite's
initializer.

tests/index/collision.c

@@ -79,8 +79,6 @@ void test_index_collision__add_with_highstage_2(void)
 {
 	git_index_entry entry;
 
-	cl_git_pass(git_repository_index(&g_index, g_repo));
-
 	memset(&entry, 0, sizeof(entry));
 	entry.ctime.seconds = 12346789;
 	entry.mtime.seconds = 12346789;