build-llvm: Don't hardcode default choices of linker/stdlib on Windows

This should help with using clangd for other mingw toolchain
installs.

build-llvm.sh

@@ -242,17 +242,6 @@ if [ -n "$COMPILER_LAUNCHER" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER_LAUNCHER=$COMPILER_LAUNCHER"
 fi
 
-if [ -n "$TARGET_WINDOWS" ]; then
-    # Custom, llvm-mingw specific defaults. We normally set these in
-    # the frontend wrappers, but this makes sure they are enabled by
-    # default if that wrapper is bypassed as well.
-    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_RTLIB=compiler-rt"
-    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_UNWINDLIB=libunwind"
-    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_CXX_STDLIB=libc++"
-    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_LINKER=lld"
-    CMAKEFLAGS="$CMAKEFLAGS -DLLD_DEFAULT_LD_LLD_IS_MINGW=ON"
-fi
-
 if [ -n "$LTO" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DLLVM_ENABLE_LTO=$LTO"
 fi