Fix build on FreeBSD (again).

Broken by commit 763f1397, which added -fno-canonical-system-headers
flag to clang (used on FreeBSD), which doesn't support it.

--
MOS_MIGRATED_REVID=113535032
diff --git a/tools/cpp/CROSSTOOL b/tools/cpp/CROSSTOOL
index b3f1ae0..0a47bb7 100644
--- a/tools/cpp/CROSSTOOL
+++ b/tools/cpp/CROSSTOOL
@@ -325,7 +325,6 @@
 
   # Anticipated future default.
   unfiltered_cxx_flag: "-no-canonical-prefixes"
-  unfiltered_cxx_flag: "-fno-canonical-system-headers"
 
   # Make C++ compilation deterministic. Use linkstamping instead of these
   # compiler symbols.