Remove dead code around use_frame_rate_api

There's no se polixy for ro.surface_flinger.use_frame_rate_api
and it cannot be used by vendors, so the code is dead. Removing
it cos there isn't a use case for disabling this API.

Bug: 199911262
Test: n/a
Change-Id: I45aabf5ced73d565a5d7adc73971f3615dde0abe