Workaround patch: Do not remove GROUP BY clause when it has
subquer(ies) in it.

remove_redundant_subquery_clauses() removes redundant GROUP BY clause
from queries in form:
  expr IN (SELECT no_aggregates GROUP BY ...)
  expr {CMP} {ALL|ANY|SOME} (SELECT no_aggregates GROUP BY ...)
This hits problems when the GROUP BY clause itself has subquer(y/ies).

This patch is just a workaround: it disables removal of GROUP BY clause
if the clause has one or more subqueries in it.

Tests:
- subselect_elimination.test has all known crashing cases.
- subselect4.result, insert_select.result are updated.
Note that in some cases results of SELECT are changed too (not just
EXPLAINs). These are caused by non-deterministic SQL: when running a
query like:

  x > ANY( SELECT col1 FROM t1 GROUP BY constant_expression)

without removing the GROUP BY, the executor is free to pick the value
of t1.col1 from any row in the GROUP BY group (denote it $COL1_VAL).
Then, it computes x > ANY(SELECT $COL1_VAL).

When running the same query and removing the GROUP BY:

   x > ANY( SELECT col1 FROM t1)

the executor will actually check all rows of t1.