[libomptarget] Deprecate old method for setting the tripcount

Previously, the tripcount was set by a push call. We moved away from
this with the new interface that added the tripcount to the kernel
arguments struct, but kept around the old interface for legacy purposes
for the LLVM 15 release. This patch removes the support for the legacy
method.

This removes the support for the old method, but does not break
backwards compatibility. This will result in applications using the old
interface being slower when run on the device.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D132885