Remove unused _TUPLE_CACHE module global from vumi.tests.utils.
Remove unused _TUPLE_CACHE module global from vumi.tests.utils.

TouK/vumi,vishwaprakashmishra/xmatrix,TouK/vumi,harrissoerja/vumi,TouK/vumi,vishwaprakashmishra/xmatrix,vishwaprakashmishra/xmatrix,harrissoerja/vumi,harrissoerja/vumi