Remove hardcoded configure macros
Remove hardcoded configure macros

Summary: Remove the `THRIFT_HAVE_SCHED_GET_PRIORITY_MIN` and `THRIFT_HAVE_SCHED_GET_PRIORITY_MAX` macros because we no longer have a `configure` script to set them and they are hardcoded. Detection of `sched_get_priority_min/max` can be done with overloading but it seems unnecessary so just remove the macros and use those functions unconditionally.

Reviewed By: zhxchen17

Differential Revision: D25796274

fbshipit-source-id: 1aafa2a3dc08b3fa1fe462007ccbece509bee4be

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