Remove "Ada Settings" node from the manual

A while back, I sent a patch to unify the Ada varsize-limit setting
with the more generic max-value-size:

https://sourceware.org/pipermail/gdb-patches/2021-September/182004.html

However, it turns out I somehow neglected to send part of the patch.
Internally, I also removed the "Ada Settings" node from the manual, as
it only documents the obsolete setting.

This patch removes this text.