Remove wasteful double-caching of settings metadata

[colemanw]

Overview

Improves efficiency by removing redundant caching and repeated calls to the same hook.

Before

Settings metadata was being cached once per domain and one extra time for good measure.
hook_civicrm_alterSettingsMetaData() was invoked even when fetching cached data, even though the hook-altered data was stored in the cache.

After

Settings metadata cached once per domain.
hook_civicrm_alterSettingsMetaData() is invoked only once per domain; as its results were being cached anyway I don't see the need to invoke it repeatedly.

Technical details

Instead of firing hook_civicrm_alterSettingsMetaData() once and then caching the results, the hook was fired once, cached, and then fired again every time metadata was loaded (potentially hundreds of times per request). In addition to being inefficient this could cause weird bugs since the data already altered by hook was then being passed into the hook to be altered again; depending on how the hook implementation was written this could give unexpected results.

[civibot]

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[colemanw]

test this please

[colemanw]

@eileenmcnaughton this is an initial cleanup toward getting the api3 Settings tests working with api4 so hopefully we can get this merged so I can keep on toward that goal.
Once this is merged I've got another REF to submit to get the 'getoptions' logic out of api3 and into core.

[eileenmcnaughton]

Looks good & we have heavy testing so I think this should be pretty safe