Don't undefine __STRICT_ANSI__ for mingw (HaxeFoundation#1112)

This was done originally to fix HaxeFoundation#785. However, since gcc 11.1.0, this
gives a warning:
__STRICT_ANSI__ seems to have been undefined; this is not supported

Due to this change:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;f=libstdc%2B%2B-v3/include/bits/c%2B%2Bconfig;h=767537a8b027bcb5807bb45b0268c5da98c2c7a0

The original issue no longer happens with mingw-w64. The original mingw
project is outdated (and 32 bit only) it is safe remove this flag now.

Author: tobil4sk

toolchain/mingw-toolchain.xml

@@ -42,7 +42,6 @@
   <flag value="-DHXCPP_BIG_ENDIAN" if="HXCPP_BIG_ENDIAN"/>
   <cppflag value="-std=c++11" if="HXCPP_CPP11" />
   <cppflag value="-std=c++17" if="HXCPP_CPP17" />
-  <cppflag value="-U__STRICT_ANSI__" />
   <flag value="-m32" unless="HXCPP_M64"/>
   <flag value="-m64" if="HXCPP_M64"/>
   <flag value="-DHXCPP_M64" if="HXCPP_M64"/>