paster_commands: remove unused imports

Remove some unused imports, using 'autoflake' followed by manual redacting.
find kallithea/lib/paster_commands/ -name "*.py" \
| xargs autoflake --remove-all-unused-imports -i

This same command could be applied gradually throughout the code base as
areas are touched. Since autoflake may be too greedy in special cases, it is
more difficult to do a big-bang action over all code.