Foreign key deletions aren't working on MySQL

[timmyomahony]

Our migrations for deleting FKs look like:

$this->dropColumn(
  "{{%matrixfieldpreview_blocktypes_config}}",
  "categoryId");

Which works OK on Postgres, but seems to cause an issue with MySQL. It seems like both the index and constraint have to be deleted separately. When I tried this though I got further errors.