Revert "introduce tainted variables and hashes"

This reverts commit d25832333c195e82cd71eb1d3311ae2dd132d95e.

Only rebuilding tainted nodes wastes too much memory space on some benchmarks.
The problem is that too many nodes may be kept unnecessarily in local tables.

Conflicts:
	library/clp/clpb.pl
ENHANCED: CLP(B): Better support for large conjunctions in sat/1.