builtin/difftool: remove unnecessary if statement

Since we already teach the `repo_config()` in "f29f1990b5
(config: teach repo_config to allow `repo` to be NULL, 2025-03-08)"
to allow `repo` to be NULL, no need to check if `repo` is NULL
before calling `repo_config()`.

Suggested-by: Patrick Steinhardt <[email protected]>
Mentored-by: Christian Couder <[email protected]>
Signed-off-by: Usman Akinyemi <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Unique-Usman

builtin/difftool.c

@@ -750,8 +750,7 @@ int cmd_difftool(int argc,
 	};
 	struct child_process child = CHILD_PROCESS_INIT;
 
-	if (repo)
-		repo_config(repo, difftool_config, &dt_options);
+	repo_config(repo, difftool_config, &dt_options);
 	dt_options.symlinks = dt_options.has_symlinks;
 
 	argc = parse_options(argc, argv, prefix, builtin_difftool_options,