Fix EZP-23599 legacy search location gateway not correctly quoting depth column

Jira issue: https://jira.ez.no/browse/EZP-23599

I was trying to create custom SortClauseHandler, in which I was joining external table which contained depth column.

Because legacy search location gateway was not correctly referencing the depth column from ezcontentobject_tree, I was getting an error stating that depth column was ambiguous.

This pull request fixes this. If you require me to create jira issue, please let me know.
Minor nitpick: Shouldn't the column/table strings be single quoted? 
Not certain on CS rules, but it seems to be the case with the other string constants.