mailutils: use system-sendmail instead of sendmailPath

system-sendmail allows all sendmail's to be auto-detected, including on
non-NixOS systems. This is, to me, a better UX than having to manually
override the sendmailPath argument.

In exchange, it is a breach of retro-compatibility. Given right now I
can't see any uses for sendmailPath other than what is supported by
system-sendmail, I didn't keep it, but it'd be possible to allow
sendmailPath to override the choice of sendmail from system-sendmail.

nixos/doc/manual/release-notes/rl-1903.xml

@@ -387,6 +387,14 @@
     root CA (for example Let's Encrypt).
    </para>
   </listitem>
+   <listitem>
+    <para>
+     <literal>mailutils</literal> now works by default when
+     <literal>sendmail</literal> is not in a setuid wrapper. As a consequence,
+     the <literal>sendmailPath</literal> argument, having lost its main use, has
+     been removed.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>