doc/ffmpeg: remove entry for -loop_output

Option doesn't exist .. hasn't for a few years now.

doc/ffmpeg.texi

@@ -1396,10 +1396,6 @@ loss).
 By default @command{ffmpeg} attempts to read the input(s) as fast as possible.
 This option will slow down the reading of the input(s) to the native frame rate
 of the input(s). It is useful for real-time output (e.g. live streaming).
-@item -loop_output @var{number_of_times}
-Repeatedly loop output for formats that support looping such as animated GIF
-(0 will loop the output infinitely).
-This option is deprecated, use -loop.
 @item -vsync @var{parameter}
 Video sync method.
 For compatibility reasons old values can be specified as numbers.