gcc: drop libstdc++ patch

This patch was introduced in 2010 to fix mingw compilers.
See c548c084bb
It no longer applies to gcc15 and all of our current gccs (13, 14 and 15)
build fine without it.

Tested the following cross-compilers:
```shell
nix-build --expr 'with (import ./. { }).pkgsCross.mingwW64; hello.override { stdenv = gcc13Stdenv; }' \
          --expr 'with (import ./. { }).pkgsCross.mingwW64; hello.override { stdenv = gcc14Stdenv; }' \
          --expr 'with (import ./. { }).pkgsCross.mingwW64; hello.override { stdenv = gcc15Stdenv; }' \
          --expr 'with (import ./. { }).pkgsCross.mingw32; hello.override { stdenv = gcc13Stdenv; }' \
          --expr 'with (import ./. { }).pkgsCross.mingw32; hello.override { stdenv = gcc14Stdenv; }' \
          --expr 'with (import ./. { }).pkgsCross.mingw32; hello.override { stdenv = gcc15Stdenv; }'
```

pkgs/development/compilers/gcc/patches/default.nix

@@ -52,10 +52,7 @@ in
 
 ## 1. Patches relevant on every platform ####################################
 
-[ ]
-# Pass the path to a C++ compiler directly in the Makefile.in
-++ optional (!lib.systems.equals targetPlatform hostPlatform) ./libstdc++-target.patch
-++ optionals noSysDirs (
+optionals noSysDirs (
   [
     # Do not try looking for binaries and libraries in /lib and /usr/lib
     ./gcc-12-no-sys-dirs.patch

pkgs/development/compilers/gcc/patches/libstdc++-target.patch

@@ -1,32 +0,0 @@
-Patch to make the target libraries 'configure' scripts find the proper CPP.
-I noticed that building the mingw32 cross compiler.
-Looking at the build script for mingw in archlinux, I think that only nixos
-needs this patch. I don't know why.
-diff --git a/Makefile.in b/Makefile.in
-index 93f66b6..d691917 100644
---- a/Makefile.in
-+++ b/Makefile.in
-@@ -266,6 +266,7 @@ BASE_TARGET_EXPORTS = \
- 	AR="$(AR_FOR_TARGET)"; export AR; \
- 	AS="$(COMPILER_AS_FOR_TARGET)"; export AS; \
- 	CC="$(CC_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CC; \
-+	CPP="$(CC_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS -E"; export CC; \
- 	CFLAGS="$(CFLAGS_FOR_TARGET)"; export CFLAGS; \
- 	CONFIG_SHELL="$(SHELL)"; export CONFIG_SHELL; \
- 	CPPFLAGS="$(CPPFLAGS_FOR_TARGET)"; export CPPFLAGS; \
-@@ -291,11 +292,13 @@ BASE_TARGET_EXPORTS = \
- RAW_CXX_TARGET_EXPORTS = \
- 	$(BASE_TARGET_EXPORTS) \
- 	CXX_FOR_TARGET="$(RAW_CXX_FOR_TARGET)"; export CXX_FOR_TARGET; \
--	CXX="$(RAW_CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX;
-+	CXX="$(RAW_CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX; \
-+	CXXCPP="$(RAW_CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS -E"; export CXX;
- 
- NORMAL_TARGET_EXPORTS = \
- 	$(BASE_TARGET_EXPORTS) \
--	CXX="$(CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX;
-+	CXX="$(CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX; \
-+	CXXCPP="$(CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS -E"; export CXX;
- 
- # Where to find GMP
- HOST_GMPLIBS = @gmplibs@