Editorial: Simplify OrdinaryFunctionCreate's _kind_ parameter

The only effect of OrdinaryFunctionCreate's _kind_ parameter
is on the [[ThisMode]] of the new function.
And although _kind_ has three possible values (~Normal~, ~Method~, ~Arrow~),
the only thing that matters is whether or not it's ~Arrow~.
So replace it with parameter _thisMode_ that has two possible values:
~lexical-this~ and ~nonlexical-this~.

Suggested by @syg in:
tc39#1562 (comment)