cindex.py: remove obsolete workaround and FIXME

clang_getCursorSpelling() doesn't assert on non-declarations any more and the
behaviour is covered by c-index tests.

Passes nosetests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211482 91177308-0d34-0410-b5e6-96231b3b80d8