pcp-dstat: drop use of list clear() as its unavailable in python2
pcp-dstat: drop use of list clear() as its unavailable in python2

Resolves QA test failures on platforms with earlier pythons (el7).

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp