Stop setting -fmsc-version in our build

This was needed back in
https://boringssl-review.googlesource.com/c/boringssl/+/21244, but
nowadays clang defaults to a higher value anyway. See
https://clang.llvm.org/docs/CommandGuide/clang.html#cmdoption-fmsc-version

Change-Id: I6d7b92ee9b9920713fe93f92f12110ec5055bbbf
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/85227
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Xiangfei Ding <xfding@google.com>
Reviewed-by: Xiangfei Ding <xfding@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 378b36e..ffe3c4c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -124,7 +124,7 @@
     # clang-cl sets different default warnings than clang. It also treats -Wall
     # as -Weverything, to match MSVC. Instead -W3 is the alias for -Wall.
     # See http://llvm.org/viewvc/llvm-project?view=revision&revision=319116
-    set(C_CXX_FLAGS "${C_CXX_FLAGS} -W3 -Wno-unused-parameter -fmsc-version=1900")
+    set(C_CXX_FLAGS "${C_CXX_FLAGS} -W3 -Wno-unused-parameter")
   else()
     if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
       # emscripten's emcc/clang does not accept the "-ggdb" flag.