doc/avconv: remove documentation for removed 'timestamp' option

doc/avconv.texi

@@ -148,18 +148,6 @@ The offset is added to the timestamps of the input files.
 Specifying a positive offset means that the corresponding
 streams are delayed by 'offset' seconds.
 
-@item -timestamp @var{time}
-Set the recording timestamp in the container.
-The syntax for @var{time} is:
-@example
-now|([(YYYY-MM-DD|YYYYMMDD)[T|t| ]]((HH[:MM[:SS[.m...]]])|(HH[MM[SS[.m...]]]))[Z|z])
-@end example
-If the value is "now" it takes the current time.
-Time is local time unless 'Z' or 'z' is appended, in which case it is
-interpreted as UTC.
-If the year-month-day part is not specified it takes the current
-year-month-day.
-
 @item -metadata[:metadata_specifier] @var{key}=@var{value}
 Set a metadata key/value pair.