cindex/test_cursor.py: no need to skip implicit decls in tests

clang was fixed some time ago to always skip "builtins and other cruft" so
tools no longer need hacks like this.

Passes nosetests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209316 91177308-0d34-0410-b5e6-96231b3b80d8