[pkg] remove leftover simplejson imports from l2db

common/src/leap/soledad/common/l2db/__init__.py

common/src/leap/soledad/common/l2db/backends/__init__.py

common/src/leap/soledad/common/l2db/backends/inmemory.py

...n/src/leap/soledad/common/l2db/backends/sqlite_backend.py

.../leap/soledad/common/l2db/remote/basic_auth_middleware.py

common/src/leap/soledad/common/l2db/remote/http_app.py

common/src/leap/soledad/common/l2db/remote/http_client.py

common/src/leap/soledad/common/l2db/remote/http_database.py

common/src/leap/soledad/common/l2db/remote/http_target.py