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.
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git-svn-id: http://core.svn.wordpress.org/trunk@30265 1a063a9b-81f0-0310-95a4-ce76da25c4cd