Revert optionality of metric (#752).

spec/ttml2.xml

@@ -17458,7 +17458,7 @@ clock-time
   : hours ":" minutes ":" seconds ( fraction | ":" frames ( "." sub-frames )? )?
 
 offset-time
-  : time-count fraction? metric?
+  : time-count fraction? metric
 
 wallclock-time
   : "wallclock(" <loc href="#style-value-lwsp">&lt;lwsp&gt;</loc>? ( date-time | wall-time | date ) <loc href="#style-value-lwsp">&lt;lwsp&gt;</loc>? ")"
@@ -17545,9 +17545,6 @@ sub-frame rate determined by the <att>ttp:subFrameRate</att> parameter as define
 ref="parameter-attribute-subFrameRate"/>.
 It is considered an error if a <emph>sub-frames</emph> term is specified
 when the <code>clock</code> time base applies.</p>
-<p>If a &lt;time-expression&gt; is expressed in terms of an
-<emph>offset-time</emph> and no <emph>metric</emph> is specified, then it is to be treated as
-if a metric of <code>s</code> (seconds) were specified.</p>
 <p>It is considered an error if the <emph>wallclock-time</emph> form of a &lt;time-expression&gt;
 is used in a <loc href="#terms-document-instance">document instance</loc> and the government time base is not <code>clock</code>.</p>
 <p>If the computed value of the governing time base is <code>smpte</code>, then