stash: stop warning about the obsolete stash.useBuiltin config setting

In 8a2cd3f (stash: remove the stash.useBuiltin setting, 2020-03-03),
we removed support for `stash.useBuiltin`, but left a warning in its
place.

After almost two years, and several major versions, it is time to remove
even that warning.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/stash.c

@@ -788,7 +788,6 @@ static int list_stash(int argc, const char **argv, const char *prefix)
 static int show_stat = 1;
 static int show_patch;
 static int show_include_untracked;
-static int use_legacy_stash;
 
 static int git_stash_config(const char *var, const char *value, void *cb)
 {
@@ -804,10 +803,6 @@ static int git_stash_config(const char *var, const char *value, void *cb)
 		show_include_untracked = git_config_bool(var, value);
 		return 0;
 	}
-	if (!strcmp(var, "stash.usebuiltin")) {
-		use_legacy_stash = !git_config_bool(var, value);
-		return 0;
-	}
 	return git_diff_basic_config(var, value, cb);
 }
 
@@ -1778,11 +1773,6 @@ int cmd_stash(int argc, const char **argv, const char *prefix)
 
 	git_config(git_stash_config, NULL);
 
-	if (use_legacy_stash ||
-	    !git_env_bool("GIT_TEST_STASH_USE_BUILTIN", -1))
-		warning(_("the stash.useBuiltin support has been removed!\n"
-			  "See its entry in 'git help config' for details."));
-
 	argc = parse_options(argc, argv, prefix, options, git_stash_usage,
 			     PARSE_OPT_KEEP_UNKNOWN | PARSE_OPT_KEEP_DASHDASH);
 

t/t3903-stash.sh

@@ -1272,7 +1272,6 @@ test_expect_success 'stash works when user.name and user.email are not set' '
 	>2 &&
 	git add 2 &&
 	test_config user.useconfigonly true &&
-	test_config stash.usebuiltin true &&
 	(
 		sane_unset GIT_AUTHOR_NAME &&
 		sane_unset GIT_AUTHOR_EMAIL &&
@@ -1323,20 +1322,6 @@ test_expect_success 'stash handles skip-worktree entries nicely' '
 	git rev-parse --verify refs/stash:A.t
 '
 
-test_expect_success 'stash -c stash.useBuiltin=false warning ' '
-	expected="stash.useBuiltin support has been removed" &&
-
-	git -c stash.useBuiltin=false stash 2>err &&
-	test_i18ngrep "$expected" err &&
-	env GIT_TEST_STASH_USE_BUILTIN=false git stash 2>err &&
-	test_i18ngrep "$expected" err &&
-
-	git -c stash.useBuiltin=true stash 2>err &&
-	test_must_be_empty err &&
-	env GIT_TEST_STASH_USE_BUILTIN=true git stash 2>err &&
-	test_must_be_empty err
-'
-
 test_expect_success 'git stash succeeds despite directory/file change' '
 	test_create_repo directory_file_switch_v1 &&
 	(