Remove a few of the new DROP-IF-EXISTS regression tests.

Commit 3301c835 broke the build farm.
Let's try to fix that.

src/test/regress/expected/drop_if_exists.out

@@ -86,12 +86,8 @@ NOTICE:  role "tg2" does not exist, skipping
 DROP GROUP tg1;
 ERROR:  role "tg1" does not exist
 -- collation
-DROP COLLATION test_collation_exists;
-ERROR:  collation "test_collation_exists" for encoding "UTF8" does not exist
 DROP COLLATION IF EXISTS test_collation_exists;
 NOTICE:  collation "test_collation_exists" does not exist, skipping
-CREATE COLLATION test_collation_exists FROM "POSIX";
-DROP COLLATION test_collation_exists;
 -- conversion
 DROP CONVERSION test_conversion_exists;
 ERROR:  conversion "test_conversion_exists" does not exist

src/test/regress/sql/drop_if_exists.sql

@@ -109,10 +109,7 @@ DROP GROUP IF EXISTS tg1, tg2;
 DROP GROUP tg1;
 
 -- collation
-DROP COLLATION test_collation_exists;
 DROP COLLATION IF EXISTS test_collation_exists;
-CREATE COLLATION test_collation_exists FROM "POSIX";
-DROP COLLATION test_collation_exists;
 
 -- conversion
 DROP CONVERSION test_conversion_exists;