Duplicate functions `yoast_rss_date` and `yoast_rss_text_limit`

[matt-bernhardt]

The two functions yoast_rss_date and yoast_rss_text_limit are defined separately in both rss-biblio.php and rss-event.php. This is inefficient - the functions should be defined in functions.php.

It also appears that yoast_rss_text_limit may be a candidate to fold into excerpt