Remove generated sql files for migration

[hanefi]

We create migration scripts such as the following:

postgresql-hll/Makefile

Lines 38 to 39 in bd1eba0

	$(EXTENSION)--2.11.sql: $(EXTENSION)--2.10.sql $(EXTENSION)--2.10--2.11.sql
	cat $^ > $@

We no longer need to generate these files, as PG >= 10 is clever enough to find migration paths automatically.

Originally posted by @hanefi in #132 (comment)