Remove some private symbols from librt

[asomers]

Summary

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.

[kib]

There are parallel exports from libc private namespace, similar to lio_listio.

I do not quite understand the original motivation for providing exported _aio (and broken) symbols from libc. IMO if removing _aio from librt, libc private export should be cleared as well.

[asomers]

In D11649#241332, @kib wrote:

There are parallel exports from libc private namespace, similar to lio_listio.

I do not quite understand the original motivation for providing exported _aio (and broken) symbols from libc. IMO if removing _aio from librt, libc private export should be cleared as well.

Good catch. I'll remove them too.

[asomers]

Remove nonexistent symbols from libc's private namespace too