mailbomb: add a comment and remove the bcc in a more pythonic way

hgext/patchbomb.py

@@ -288,7 +288,8 b' def patchbomb(ui, repo, *revs, **opts):'
             fp.close()
         else:
             ui.status('Sending ', m['Subject'], ' ...\n')
-            m.__delitem__('bcc')
+            # Exim does not remove the Bcc field
+            del m['Bcc']
             mail.sendmail(sender, to + bcc + cc, m.as_string(0))
 
 cmdtable = {