Kill get_fast/get_weak_fast Singletonn API
Kill get_fast/get_weak_fast Singletonn API

Summary: After D1827390 regular get and get_weak is comparable to Meyers and static singletons, so there's no need to keey _fast APIs.

Test Plan: benchmark && fbmake

Reviewed By: @​mshneer

Differential Revision: D1843219
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