[css-rhythm] Revert the addition of 'none' to 'line-height-step'

As per the discussion in #1151. The reason we added is gone, and we can add it back if the reason came back.

css-rhythm-1/Overview.bs

@@ -350,14 +350,14 @@ Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
 
 	<pre class='propdef'>
 	Name: line-height-step
-	Value: none | <<length>>
-	Initial: none
+	Value: <<length>>
+	Initial: 0
 	Applies to: block containers
 	Inherited: yes
 	Animatable: no
 	Percentages: N/A
 	Media: visual
-	Computed Value: the absolute length, 0 for none
+	Computed Value: the absolute length
 	</pre>
 
 This property defines the <dfn>step unit</dfn> for line box heights.
@@ -392,7 +392,6 @@ by making heights of all lines an integer multiple of the <a>step unit</a>.
 	<p class="caption">Rounding up the computed line box height.
 	</div>
 
-	<div class="issue">''line-height-step/none'' and ''0'' are equivalent. Is this ok?</div>
 	<div class="issue">Should this be animatable?
 	There doesn't seem to be use cases but needed for consistency?</div>