release docs:  mention PL/pgSQL as supporting event triggers in PG 9.3
Remove unused regression test files.

euc_* and mule_internal test cases were identical to the ones in
src/test/mb. sql_ascii didn't exist elsewhere, but has been broken since
2001, and doesn't seem very interesting anyway. drop.sql hasn't been used
since 2000, when regress.sh was removed.