Remove mentions of ENOSYS added in d97e44784bb5

aio(4) is a hard requirement in the kernel as of f3215338ef82. The
scenario that the patch was submitted for is no longer possible.

This isn't a straight up revert since the previous change also addressed
some minor issues.

PR: 190942
Reported by: asomers
MFC after: 2 weeks
MFC with: d97e44784bb5
Fixes: d97e44784bb5 ("aio_*(2): mention ENOSYS under ERRORS")
Differential Revision: https://reviews.freebsd.org/D49541

(cherry picked from commit e24279e0f9e28ba0c1920cb539fc357568790c0a)