[ios] Only create native session restoration cache object when enabled

Mark the native session restoration cache feature as incompatible with
the session restoration optimisation (as optimised code will manage the
native session directly).

Ensure that the native session restoration cache objects are created
only when the feature is enabled and remove the unit test checking the
objects are no-op when the feature is disabled (as they now assert the
feature is enabled when constructed).

Remove code to support iOS 14.

Bug: 1264451
Change-Id: I629d7d4cd4da7885d564717fe7350a5d7fbe3b50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4519263
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Federica Germinario <fedegermi@google.com>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1142647}