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

Merges [25954] to the 3.7 branch.

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.


git-svn-id: https://develop.svn.wordpress.org/branches/3.7@25955 602fd350-edb4-49c9-b593-d223f7449a82