Remove validation code and non-vectored read path

[VladLazar]

Now that https://github.com/neondatabase/neon/issues/7381 is done, the vectored read path is being used everywhere. At this point we can remove the non-vectored read path and all adjacent validation code.

Also update the defaults and consider https://github.com/neondatabase/neon/pull/7939

[problame]

Please hold off with this until #6967 is resolved.

See comment https://github.com/neondatabase/neon/issues/6967#issuecomment-2158477818 for details.

[VladLazar]

Please hold off with this until #6967 is resolved.

See comment #6967 (comment) for details.

ack