Don't enable cron by default

The rationale for disabling this is: 1) systemd timers are better; 2)
it gets rid of one usually unnecessary process, which makes containers
more light-weight.

Note that cron is still enabled if services.cron.systemCronJobs is
non-empty, so this only matters if you have no declarative cron jobs
but do have user cron jobs.

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

@@ -36,7 +36,7 @@
 
 </para>
 
-<para>Following new services were added since the last release:
+<para>The following new services were added since the last release:
 
 <itemizedlist>
 <listitem><para><literal>brltty</literal></para></listitem>
@@ -50,6 +50,12 @@ following incompatible changes:
 
 <itemizedlist>
 
+<listitem><para><command>cron</command> is no longer enabled by
+default, unless you have a non-empty
+<option>services.cron.systemCronJobs</option>. To force
+<command>cron</command> to be enabled, set
+<option>services.cron.enable = true</option>.</para></listitem>
+
 <listitem><para>Steam now doesn't need root rights to work. Instead of using
 <literal>*-steam-chrootenv</literal>, you should now just run <literal>steam</literal>.
 <literal>steamChrootEnv</literal> package was renamed to <literal>steam</literal>,