Remove bogus step from test_decoding isolation tests.

Commit 43b4a168 made the isolation
tester reject duplicate step names, and it turns out that the
test_decoding module's concurrent_ddl_dml isolation test has a
duplicate name.  I think the second definition isn't actually getting
used, so just remove it.

Per buildfarm.

contrib/test_decoding/specs/concurrent_ddl_dml.spec

@@ -38,7 +38,6 @@ step "s2_alter_tbl2_float" { ALTER TABLE tbl2 ALTER COLUMN val2 TYPE float; }
 step "s2_alter_tbl2_char" { ALTER TABLE tbl2 ALTER COLUMN val2 TYPE character varying; }
 step "s2_alter_tbl2_text" { ALTER TABLE tbl2 ALTER COLUMN val2 TYPE text; }
 step "s2_alter_tbl2_boolean" { ALTER TABLE tbl2 ALTER COLUMN val2 TYPE boolean; }
-step "s2_alter_tbl2_text" { ALTER TABLE tbl2 ALTER COLUMN val2 TYPE boolean; }
 
 step "s2_alter_tbl2_add_int" { ALTER TABLE tbl2 ADD COLUMN val3 INTEGER; }
 step "s2_alter_tbl2_add_float" { ALTER TABLE tbl2 ADD COLUMN val3 FLOAT; }