Remove some uses of the GCC __nonnull() attribute.

While the checks are considered useful, the attribute does dangerous
optimizations, removing NULL checks where they can be needed. Remove the
uses of this attribute introduced in r281130: the changes were inspired on
Google's bionic where this attribute is not used anymore.

The __nonnull() attribute will be deprecrated from our headers and
replaced with the Clang _Nonnull qualifier in the future.

MFC after: 3 days