Remove Symbol.map entries for old AIO system calls for FreeBSD 6 compat.

These entries should have never been present since they only exist for
compat with FreeBSD 6.x (and older) binaries. This was missed in r296572.
Technically this breaks the ABI by removing versioned symbols. However,
no binaries should be linked against these symbols. No release has
shipped with a header that contained a prototype for these functions.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D5615