Remove useless 'limit' bugbuster test.

We have "LIMIT 0" clauses in other tests.

src/test/regress/bugbuster/expected/limit.out

-drop table if exists test;
-create table test (a integer, b integer);
-NOTICE:  Table doesn't have 'DISTRIBUTED BY' clause -- Using column named 'a' as the Greenplum Database data distribution key for this table.
-HINT:  The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew.
-select * from test limit 0;
- a | b 
----+---
-(0 rows)
-

src/test/regress/bugbuster/known_good_schedule

@@ -42,7 +42,6 @@ ignore: gpcheckmirrorseg
 test: AOCO_Compression2
 test: schema_topology
 test: gpmgmt1
-test: limit
 test: case
 test: bkup_pg
 test: bkup_gpcrondump

src/test/regress/bugbuster/sql/limit.sql

-drop table if exists test;
-create table test (a integer, b integer);
-select * from test limit 0;