Resolve invalid use of a regexp in regexp-opt.

cc-fonts.el (c-complex-decl-matchers): Add in special detection for a
java annotation.

cc-engine.el (c-forward-decl-or-cast-1): Add in special detection for a
java annotation.

cc-langs.el (c-prefix-spec-kwds-re): Remove the special handling for
java.
(c-modifier-kwds): Remove the regexp "@[A-za-z0-9]+".