var lineIndex = richTextBox1.GetLineFromCharIndex(richTextBox1.SelectionStart);
var endIndex = richTextBox1.GetFirstCharIndexFromLine(lineIndex + 1);
if (endIndex == -1)
   endIndex = richTextBox1.Text.Length;

richTextBox1.Text = richTextBox1.Text.Remove(richTextBox1.SelectionStart, endIndex - richTextBox1.SelectionStart);