Remove compiler warning options that conflict with C99

Summary:
These were removed in the previous version of the C99 diff:
https://reviews.facebook.net/D15639

...but these changes were moved upstream, and were lost when I refactored
the cmake defs in the WebScaleSQL rebase to 5.6.22.  I will squash this
commit with that one in future branches.

Test Plan:
The async client diff won't build without this fix.  With it, it does.

Reviewers: inaam-rana, jeremycole, pivanof

Reviewed By: pivanof

Differential Revision: https://reviews.facebook.net/D30429