make syncdata attach only committees to laws (not plenum) 

to fix this bug: https://groups.google.com/forum/?fromgroups=#!topic/oknesset-dev/Mzamy4UwiRw

after merging need to remove existing plenum meetings from laws
can be done manually because there should only be a few of these
# get all bills with plenum meetings in them:
first=Bill.objects.filter(first_committee_meetings__committee__type='plenum')
second=Bill.objects.filter(second_committee_meetings__committee__type='plenum')
# for each bill:
# b=first[0]
b.first_committee_meetings.remove(b.first_committee_meetings.filter(committee__type='plenum'))
# b=second[0]
b.second_committee_meetings.remove(b.second_committee_meetings.filter(committee__type='plenum'))