AIX: removing the hard coded gnu++11 flag

Recent CLs on V8 have been starting to use c++14 features,
such as this: https://chromium-review.googlesource.com/c/v8/v8/+/1786284

Having the hard coded "gnu++11" flag causes the compilation to fail on AIX.

Change-Id: Iffd74e3aed00711c8c344ea278b9533079f52ffa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789965Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694497}

build/config/aix/BUILD.gn

@@ -35,7 +35,6 @@ config("compiler") {
   ]
 
   cflags_cc = [
-    "-std=gnu++11",
     "-fno-rtti",
     "-fno-exceptions",
     "-Wno-narrowing",