Remove needless quote from choice values

[syohex]

Development Emacs generates like the following warnings.

php.el:58:12: Warning: defcustom for ‘php-manual-url’ has syntactically odd
    type ‘'(choice (const :tag English 'en) (const :tag Brazilian Portuguese
    'pt_BR) (const :tag Chinese (Simplified) 'zh) (const :tag French 'fr)
    (const :tag German 'de) (const :tag Japanese 'ja) (const :tag Romanian
    'ro) (const :tag Russian 'ru) (const :tag Spanish 'es) (const :tag Turkish
    'tr) (string :tag PHP manual URL))’

[zonuexe]

@syohex Thanks!