Remove stray mentions of mbstring.func_overload

This feature has been completely removed.

Closes GH-6688.

Author: MaxSem

ext/mbstring/tests/ini_mbstring_invalid.phpt

@@ -10,7 +10,6 @@ mbstring.http_input=UTF-8,HTTP_INPUT,ASCII
 mbstring.http_output=HTTP_OUTPUT
 mbstring.http_output_conv_mimetypes=UNKNOWN_MIME_TYPE_OUTPUT
 mbstring.substitute_character=U+3000,NON_EXISTING_CHARACTER,JIS+7E7E
-mbstring.func_overload=BOOL_OVERLOAD
 mbstring.encoding_translation=BOOL_TRANSLATION
 mbstring.strict_detection=BOOL_STRICT_DETECTION
 --FILE--

ext/mbstring/tests/mb_str_split_jp.phpt

@@ -4,7 +4,6 @@ mb_str_split() tests for the japanese language
 <?php extension_loaded('mbstring') or die('skip mbstring not available'); ?>
 --INI--
 output_handler=
-mbstring.func_overload=0
 --FILE--
 <?php
 ini_set('include_path','.');

ext/mbstring/tests/mb_str_split_ru.phpt

@@ -4,7 +4,6 @@ mb_str_split()  tests for the russian language
 <?php extension_loaded('mbstring') or die('skip mbstring not available'); ?>
 --INI--
 output_handler=
-mbstring.func_overload=0
 --FILE--
 <?php
 ini_set('include_path','.');

ext/mbstring/tests/mb_str_split_utf8_utf16.phpt

@@ -4,7 +4,6 @@ mb_str_split() tests UTF-8 illegal chars & UTF-16 surrogate pairs
 <?php extension_loaded('mbstring') or die('skip mbstring not available'); ?>
 --INI--
 output_handler=
-mbstring.func_overload=0
 --FILE--
 <?php
 ini_set('include_path','.');

run-tests.php

@@ -871,7 +871,6 @@ function write_information(): void
         'tidy' => ['tidy.clean_output=0'],
         'zlib' => ['zlib.output_compression=Off'],
         'xdebug' => ['xdebug.mode=off'],
-        'mbstring' => ['mbstring.func_overload=0'],
     ];
 
     foreach ($info_params_ex as $ext => $ini_overwrites_ex) {