Don't force child_of=0 for non-hierarchical taxonomies in `get_terms()`.

This forcing appears to have been introduced to save unnecessary queries, but
(a) in some cases it appeared to be causing *more* queries, and (b) it caused
incorrect results when the 'exclude_tree' param of `get_terms()` called
`get_terms()` on each item in the tree using the 'child_of' param.

Fixes #30275.

git-svn-id: https://develop.svn.wordpress.org/trunk@30265 602fd350-edb4-49c9-b593-d223f7449a82