Compile in whatever C++ version is default.

There's no need to force C++17 (to gain C++17 features) since Chromium
has long built in C++17 mode, and in fact this hinders moving to C++20.
Use the version of C++ that the surrounding environment defaults to.

Bug: chromium:1284275
Change-Id: Ia4aade7891e42fe39d55e8049fdc84a8b2e6e51e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/70769
Commit-Queue: Peter Kasting <pkasting@google.com>
Presubmit-Ready: Shahbaz Youssefi <syoussefi@google.com>
Tested-by: Peter Kasting <pkasting@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index da23872..b405f51 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -41,15 +41,8 @@
                     # of signed shorts.
       ]
     }
-
-    cflags_cc = [ "/std:c++17" ]
-  } else {
-    cflags_cc = [ "-std=c++17" ]
-    cflags_objcc = [ "-std=c++17" ]
-
-    if (!is_debug) {
-      cflags += [ "-Os" ]
-    }
+  } else if (!is_debug) {
+    cflags += [ "-Os" ]
   }
 
   if (build_with_chromium) {