Use sigInst.methInstCount != 0 to detect generic methods (#11780)

Use sigInst.methInstCount != 0 to detect generic methods (#11780)

`methInstCount` is the thing that RyuJIT uses to detect generic virtual
methods in another place. `CORINFO_CALLCONV_GENERIC` reporting was added
because nobody knew about `methInstCount`. We already report
`methodInstCount` in CoreRT. We can delete a couple of lines on the
CoreRT side by switching to this scheme.