Remove old execLaterNative code path

[wch]

Some time after #104 is released and the new version of later is in widespread use, we should modify inst/include/later.h as described here:

later/inst/include/later.h

Lines 88 to 95 in 97462f4

	// Note 2019-09-11: The above code in this function is here just in case a
	// package built with this version of later.h is run with an older version
	// of the later DLL which does not have the execLaterNative2 function. In
	// the next release of later, after we are confident that users have
	// installed the newer later DLL which has execLaterNative2, it should be
	// safe to replace the code in this function with just this:
	//
	// later(func, data, secs, GLOBAL_LOOP);