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.
We usually use single quotes, but also double quotes. It's not important.