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