Remove some private symbols from librt

Private functions like __aio_read and _aio_read were exposed in
FBSDprivate_1.0 by r169090, even though they've never been used outside of
librt. Also, remove some weak references from r156136 that have never
resolved.

Reviewed by: kib
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D11649