[OpenMP] Remove unused variables from libomp code

Several variables were left unused as a result of different patches removing
their use.

Two variables have some use:
`poll_count` is used by the KMP_BLOCKING macro only under certain conditions.
Adding (void) to tell the compiler to ignore the unused variable.

`padding` is a dummy stack allocation with no intent to be used. Also adding
(void) to make the compiler ignore the unused variable.

Differential Revision: https://reviews.llvm.org/D104303