[OpenMP][libomptarget] Remove unused virtual functions in GenericKernelTy

The virtual functions getDefaultNumBlocks and getDefaultNumThreads from the kernels are
only forwarding the call to the generic device's ones. This patch removes those two
functions from the kernels (and their derived ones). Now calls are made to the device's
functions directly.

Differential Revision: https://reviews.llvm.org/D156905