Remove redundant /utf-8 compile option

This was a partial fix and the characters in mbstring_arginfo.h were
converted/changed in PHP-8.4 in the meantime.

Author: petk

cmake/ext/mbstring/CMakeLists.txt

@@ -226,11 +226,6 @@ endif()
 
 target_compile_definitions(php_mbstring PRIVATE ZEND_ENABLE_STATIC_TSRMLS_CACHE)
 
-# The mbstring_arginfo.h has some special characters.
-if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
-  set_property(SOURCE mbstring.c PROPERTY COMPILE_OPTIONS /utf-8)
-endif()
-
 target_include_directories(
   php_mbstring
   PRIVATE