Simplify workaround for Erlang backend.

The bug avoided by the preceding commit is that the Erlang backend can
produce code for cc_multi_equal(!IO) which refers to a non-existent
dummy IO variable.

library/thread.m:
	Replace `cc_multi_equal' by a disjunction to convince the
	compiler that `spawn_context_2' is non-deterministic.

	Delete the Erlang foreign proc.