SLICC: Remove WakeUp* import calls from ast/__init__.py

SLICC: Remove WakeUp* import calls from ast/__init__.py
I had recently committed a patch that removed the WakeUp*.py files from the
slicc/ast directory. I had forgotten to remove the import  calls for these
files from slicc/ast/__init__.py. This resulted in error while running
regressions on zizzer. This patch does the needful.