bpo-28845: Clean up known issues for AIX (GH-1670)

Clean up `Misc/README.AIX` for addressed known issues.

- Issues that have been marked fixed: #11184, #11185
- Issues resolved by new AIX version: #1745108
- Issues resolved, but not yet marked fixed/closed: #11188

Signed-off-by: Eric N. Vander Weele <ericvw@gmail.com>