Revert "[libomptarget] [amdgpu] Fix default setting of max flat workgroup size"

This reverts commit 2240b41e.
A value of 0 for KernDescVal WG_Size implies it is unknown, so it should be
set to the default. The above change was made without this assumption.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D105250