Cannot drop column : needed in a foreign key constraint
Having a look at MySql docs I've found a warning about foreign_key_keys:


  Warning
  With foreign_key_checks=0, dropping an index required by a foreign key constraint places the table in an inconsistent state and causes the foreign key check that occurs at table load to fail. To avoid this problem, remove the foreign key constraint before dropping the index (Bug #70260).


IMHO you should drop FOREIGN KEY before DROP the COLUMN.

ALTER TABLE `user` DROP FOREIGN KEY `FK_G38T6P7EKUXYWH1`;
ALTER TABLE `user` DROP COLUMN `region_id`;


I've set up a rextester example, check it here.