Remove WarnUnsafe from the standard library build until we're ready to deal with the warnings

Author: DougGregor

stdlib/public/core/CMakeLists.txt

@@ -318,7 +318,6 @@ list(APPEND swift_stdlib_compile_flags "-enable-experimental-feature" "Macros")
 list(APPEND swift_stdlib_compile_flags "-enable-experimental-feature" "FreestandingMacros")
 list(APPEND swift_stdlib_compile_flags "-enable-experimental-feature" "Extern")
 list(APPEND swift_stdlib_compile_flags "-enable-experimental-feature" "BitwiseCopyable")
-list(APPEND swift_stdlib_compile_flags "-enable-experimental-feature" "WarnUnsafe")
 
 if("${SWIFT_NATIVE_SWIFT_TOOLS_PATH}" STREQUAL "")
   set(swift_bin_dir "${CMAKE_BINARY_DIR}/bin")