Dead regression tests

There are a few .sql files in src/test/regress/sql/, that AFAICS are not
used for anything:

drop.sql

Looking at the history, this was used for some kind of cleanup in the
ancient regress.sh script. It has been unused since Nov 2000, when
regress.sh was removed.

euc_cn.sql
euc_jp.sql
euc_kr.sql
euc_tw.sql
mule_internal.sql

These are duplicates of the scripts in src/test/mb/sql/.

sql_ascii.sql

This is similar to the euc_* and mule_internal tests, but is not present
in src/test/mb/sql. Looking at the expected output, had this test ever
been run, it would've failed since at least 2001, when the error message
memorized in the expected output, "Relation '%s' does not exist", was
changed in the code.

hs_primary_extremes.sql

This was introduced as part of the hot standby patch. I don't see any
reference to how it's supposed to be used. Simon?

Barring objections, I'm going to just remove all of the above, except
for hs_primary_extremes.sql.

- Heikki