Remove an unneeded OS!=win now that update.py is used.

Follow-up to https://codereview.chromium.org/1495653002, and makes it easier
to maybe have a clang/win build of v8 at some point in the future.

BUG=none
LOG=n

Review URL: https://codereview.chromium.org/1549053002

Cr-Commit-Position: refs/heads/master@{#33037}

build/standalone.gypi

@@ -415,7 +415,7 @@
       ['clang==0', {
         'cflags+': ['-Wno-sign-compare',],
       }],
-      ['(clang==1 or host_clang==1) and OS!="win"', {
+      ['clang==1 or host_clang==1', {
         # This is here so that all files get recompiled after a clang roll and
         # when turning clang on or off.
         # (defines are passed via the command line, and build systems rebuild