Remove assertCues() helper methods from WebvttDecoderTest

These make the interesting bits of each assertion harder to follow imo.

Also remove all the assertWithMessage() calls at the same time, Olly
convinced me these are rarely useful since you can click from the stack
trace to the failing line in the IDE.

PiperOrigin-RevId: 288470704

library/core/src/test/assets/webvtt/with_positioning

@@ -8,12 +8,12 @@ This is the first subtitle.
 NOTE Wrong position provided. It should be provided as
 NOTE Wrong position provided. It should be provided as
 a percentage value
 a percentage value
 
 
-00:02.345 --> 00:03.456 position:10 align:end size:35%
+00:02.345 --> 00:03.456 position:10 align:end
 This is the second subtitle.
 This is the second subtitle.
 
 
 NOTE Line as percentage and line alignment
 NOTE Line as percentage and line alignment
 
 
-00:04.000 --> 00:05.000 line:45%,end align:middle size:35%
+00:04.000 --> 00:05.000 line:45%,end align:middle
 This is the third subtitle.
 This is the third subtitle.
 
 
 NOTE Line as absolute negative number and without line alignment.
 NOTE Line as absolute negative number and without line alignment.
@@ -23,10 +23,10 @@ This is the fourth subtitle.
 
 
 NOTE The position and positioning alignment should be inherited from align.
 NOTE The position and positioning alignment should be inherited from align.
 
 
-00:07.000 --> 00:08.000 align:right
+00:08.000 --> 00:09.000 align:right
 This is the fifth subtitle.
 This is the fifth subtitle.
 
 
 NOTE In newer drafts, align:middle has been replaced by align:center
 NOTE In newer drafts, align:middle has been replaced by align:center
 
 
-00:10.000 --> 00:11.000 line:45%,end align:center size:35%
+00:10.000 --> 00:11.000 align:center
 This is the sixth subtitle.
 This is the sixth subtitle.