Clean up unused module import and variables assigned to, but never used.

Reported via pyflakes:
libparse/wscript:26: local variable 'srcnode' is assigned to but never used
ntpfrob/wscript:2: local variable 'srcnode' is assigned to but never used
tests/wscript:3: local variable 'bldnode' is assigned to but never used
wafhelpers/test.py:8: local variable 'lst' is assigned to but never used
wafhelpers/waf.py:3: 'waflib.Task.Task' imported but unused