Undoing (part of) a change from 828ad796.

c78d279e by gilbertgede has since imported zip_longest into
compatibility, so my change is redundant.

	modified:   compatibility.py
Remove counting_decrement() method from MultisetPartitionTraverser

It turns out that the check in counting_decrement for a part being
present in the cache never succeeds (at least in the cases I have
tested), so calling the existing decrement_part() method directly
results in shorter code, and is a bit faster.

	modified:   sympy/utilities/enumerative.py