[compiler-rt] Clean up explicit -std=c++17 addition

This was superseded by 5737f6a. Clean
it up as suggested by Mike Hommey in https://reviews.llvm.org/D131937.

Author: smeenai

compiler-rt/CMakeLists.txt

@@ -300,8 +300,6 @@ if(COMPILER_RT_ENABLE_WERROR)
   append_string_if(COMPILER_RT_HAS_WX_FLAG /WX CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 endif()
 
-append_string_if(COMPILER_RT_HAS_STD_CXX17_FLAG -std=c++17 CMAKE_CXX_FLAGS)
-
 # Emulate C99 and C++11's __func__ for MSVC prior to 2013 CTP.
 if(NOT COMPILER_RT_HAS_FUNC_SYMBOL)
   add_definitions(-D__func__=__FUNCTION__)

compiler-rt/cmake/config-ix.cmake

@@ -81,7 +81,6 @@ check_cxx_compiler_flag(-fvisibility=hidden  COMPILER_RT_HAS_FVISIBILITY_HIDDEN_
 check_cxx_compiler_flag(-frtti               COMPILER_RT_HAS_FRTTI_FLAG)
 check_cxx_compiler_flag(-fno-rtti            COMPILER_RT_HAS_FNO_RTTI_FLAG)
 check_cxx_compiler_flag("-Werror -fno-function-sections" COMPILER_RT_HAS_FNO_FUNCTION_SECTIONS_FLAG)
-check_cxx_compiler_flag(-std=c++17           COMPILER_RT_HAS_STD_CXX17_FLAG)
 check_cxx_compiler_flag(-ftls-model=initial-exec COMPILER_RT_HAS_FTLS_MODEL_INITIAL_EXEC)
 check_cxx_compiler_flag(-fno-lto             COMPILER_RT_HAS_FNO_LTO_FLAG)
 check_cxx_compiler_flag(-fno-profile-generate COMPILER_RT_HAS_FNO_PROFILE_GENERATE_FLAG)