xgettext: JavaScript: Make more use of mixed_string_buffer.
* gettext-tools/src/x-javascript.c (struct unicode_string_buffer): Remove type.
(init_unicode_string_buffer, unicode_string_buffer_append_unicode_grow,
unicode_string_buffer_append_unicode, unicode_string_buffer_result,
free_unicode_string_buffer): Remove functions.
(comment_buffer, comment_start, comment_at_start, comment_add,
comment_line_end): Use mixed_string_buffer API.
