Remove error-prone mlir::ExecutionEngine::invoke overload.

I just spent a bunch of time debugging a mysterious bug that ended being due to my SmallVector getting passed to the Args&... overload instead of the MutableArrayRef overload, with disastrous results.

I appreciate the intent of this API, but for a function that does a bunch of unsafe casts, adding in potential overload confusion is just too much C++ footgun. If we end up needing this functionality, having something like a separate `packArgs(Args&...) -> SmallVector` overload would be preferable.

Turns out this API is unused and untested (even out of tree as far as I can tell, modulo the optional passing of no args to the other invoke as I fixed in this patch), so it's an easy fix -- just delete it and touch up the other overload.

Differential Revision: https://reviews.llvm.org/D80607