Remove unused parts of unused query text in GROUP BY handler processing.

ColumnStore VTABLE processing mode heavily depends on query text rewrite. Connector code uses strings for parts of the rewritten query text. GROUP BY Handler doesn't need this rewrites though GBH processing part still contains unused and sometimes empty parts of rewritten query. This could cause unexpected server crashes for queries with complex expression in a projection list like described in the bug linked.
One has to remove all the relevant sub-parts of the rewritten query text.