[OpenMP] Remove checkXXXX device runtime functions

We had multiple functions to determine the execution mode (SPMD/Generic)
and runtime status (initialized/uninitialized) but that just increased
complexity without a real benefit. Especially with D102307 in mind it
is helpful to reduce the dependence on the `ident_t` flags.

Differential Revision: https://reviews.llvm.org/D105586