Not really anything to do with jQuery, but if you want to trim a pattern from a string, then use a regular expression:
<textarea id="ta0"></textarea>
<button onclick="
var ta = document.getElementById('ta0');
var text = 'some<br>text<br />to<br/>replace';
var re = /<br *\/?>/gi;
ta.value = text.replace(re, '\n');
">Add stuff to text area</button>