Theme Editor: Remove unused `$has_templates` boolean.

The variable is a remnant from when the Theme Editor still had the "Templates" heading and is unused in the current logic.

The condition it was a part of would always evaluate to true for a child theme, regardless of the `$has_templates` value.

Props dboy1988.
Fixes #50199.
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git-svn-id: https://core.svn.wordpress.org/trunk@47620 1a063a9b-81f0-0310-95a4-ce76da25c4cd