Address code quality issues

Author: macflo8

message_ix_models/model/material/build.py

@@ -13,7 +13,6 @@
 from message_ix_models.model.material.data_generic import gen_data_generic
 from message_ix_models.model.material.data_methanol import gen_data_methanol
 from message_ix_models.model.material.data_petro import gen_data_petro_chemicals
-from message_ix_models.model.material.data_power_sector import gen_data_power_sector
 from message_ix_models.model.material.data_steel import gen_data_steel
 from message_ix_models.model.material.data_util import (
     add_ccs_technologies,