Update media-alternative-prerecorded.html (#4326)

Only one note exists in the 2.1 and 2.2 versions of the 1.2.8
Understanding document; however, it contains multiple paragraphs, which
are enumerated as separate notes in 1.2.3 and 1.2.5 (and in the 1.2.8
Understanding in the 2.0 version). The first paragraph of 1.2.8, which
corresponds to note 1 in 1.2.3 and 1.2.5, is irrelevant to 1.2.8. By
removing this paragraph, 1.2.8's note is reduced to covering a single
concept, which results in an unnumbered note.

Author: mbgower

understanding/20/media-alternative-prerecorded.html

@@ -56,13 +56,6 @@ Intent of Media Alternative (Prerecorded)
 
       <div class="note">
 
-         <p>
-            For 1.2.3, 1.2.5, and 1.2.7, if all of the information in the video track is already
-            provided in the audio track, no audio description is necessary. 
-            
-            
-         p>
-         
          <p>
             1.2.3, 1.2.5, and 1.2.8 overlap somewhat with each other. This is to give the author
             some choice at the minimum conformance level, and to provide additional requirements