[lit] Remove LitTestCase

From the docs: `class LitTestCase(unittest.TestCase)`
LitTestCase is an adaptor for providing a 'unittest' compatible
interface to 'lit' tests so that we can run lit tests with standard
python test runners.

It does not seem to be used anywhere.

Differential Revision: https://reviews.llvm.org/D58264

llvm-svn: 354188