cmake: Removed CMake flags for controlling C standard

- `CMAKE_C_EXTENSIONS`: Enable/disable compiler-specific extensions
- `CMAKE_C_STANDARD`: Set C standard (C99, C11)
- These variables (as set in mbedtls) set the C standard to C99
  for ESP-IDF as well, resulting in build errors related to
  `asm volatile`

CMakeLists.txt

@@ -172,8 +172,6 @@ string(REGEX MATCH "Clang" CMAKE_COMPILER_IS_CLANG "${CMAKE_C_COMPILER_ID}")
 
 include(CheckCCompilerFlag)
 
-set(CMAKE_C_EXTENSIONS OFF)
-set(CMAKE_C_STANDARD 99)
 
 if(CMAKE_COMPILER_IS_GNU)
     # some warnings we want are not available with old GCC versions