Update expected results for collate.linux.utf8 regression test.

I believe this changed as a consequence of commit 54baa4813: trying to
clone the "C" collation now produces a true clone with collencoding -1,
hence the error message if it's duplicate no longer specifies an encoding.

Per buildfarm member crake, which apparently hadn't been running this
test for the last few weeks.

src/test/regress/expected/collate.linux.utf8.out

@@ -996,9 +996,9 @@ BEGIN
 END
 $$;
 CREATE COLLATION test0 FROM "C"; -- fail, duplicate name
-ERROR:  collation "test0" for encoding "UTF8" already exists
+ERROR:  collation "test0" already exists
 CREATE COLLATION IF NOT EXISTS test0 FROM "C"; -- ok, skipped
-NOTICE:  collation "test0" for encoding "UTF8" already exists, skipping
+NOTICE:  collation "test0" already exists, skipping
 CREATE COLLATION IF NOT EXISTS test0 (locale = 'foo'); -- ok, skipped
 NOTICE:  collation "test0" for encoding "UTF8" already exists, skipping
 do $$