[Libomptarget] Use new tripcount argument in the runtime.

The previous patch added an argument to the `__tgt_target_kernel`
runtime function which includes the tripcount used for the loop clause.
This was originally passed in via the `__kmpc_push_target_tripcount`
function. Now we move this logic to the kernel launch itself and remove
the need for the push function.

Depends on D128816

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D128817