Query stopwords: Only eliminate single A-Z letters as search terms.

Stop trying to match any single letters that are not East Asian characters, as this requires PCRE with UTF-8 support; and because it doesn't actually work.

fixes #25709.

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git-svn-id: http://core.svn.wordpress.org/trunk@25913 1a063a9b-81f0-0310-95a4-ce76da25c4cd