Remove enableEagerMainQueueModulesOnIOS, make eager main-queue setup always-on (#56933)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56933

The `enableEagerMainQueueModulesOnIOS` runtime gate has finished rollout and is no longer needed. Drop the flag from the feature-flag config, regenerate the per-language accessors, and inline the previously-gated body in `RCTInstance._start` so the eager main-queue module setup always runs. Also drops the now-dead override and trims the test that exercised the flag-off branch.

After this change, when `RCTInstance` starts up, it always consults `RCTInstanceDelegate.unstableModulesRequiringMainQueueSetup`, initializes the returned modules on the main queue, and blocks the JS thread on that completion before evaluating the bundle.

Changelog:
[iOS][Changed] - Native modules listed in `unstableModulesRequiringMainQueueSetup` are now always initialized eagerly on the main queue during React Native init; the previous `enableEagerMainQueueModulesOnIOS` opt-in flag has been removed.

Reviewed By: sammy-SC

Differential Revision: D105954235

fbshipit-source-id: a662d43e606f215647fb86a87a62fe123c003360