[OMPT] Simplify control variable logic for OMPT

Prior to this change, OMPT had a status flag ompt_status, which could take
several values. This was due to an earlier OMPT design that had several levels
of enablement (ready, disabled, tracking state, tracking callbacks). The
current OMPT design has OMPT support either on or off.
This revision replaces ompt_status with a boolean flag ompt_enabled, which
simplifies the runtime logic for OMPT.

Patch by John Mellor-Crummey

Differential Revision: http://reviews.llvm.org/D12999