BUG Fixing SQLQuery::aggregate() adding ORDER BY when no limit.

DataQuery::initialiseQuery() will add a default sort to a query,
and when calling up an aggregate it will make a query like this
which doesn't make sense:

SELECT MAX("LastEdited") FROM "Member" ORDER BY "ID"

In this case there is no need to add the ORDER BY, and it will
break databases like MSSQL in cases such as GenericTemplateGlobalProvider
which provides a default List() function for adding aggregates
into SSViewer template cacheblocks.

If we add a limit, however, then it does make sense:

SELECT MAX("LastEdited") FROM "Member" ORDER BY "ID" LIMIT 10

This fixes SQLQuery::aggregate() to NOT add an ORDER BY to an
aggregate call if there is no limit.

References: #1712 silverstripe/silverstripe-mssql#12