doc/filters: remove outdated comments

Audio API should not be considered unstable any longer, and mentioning
VHOOK is confusing, as it was removed 4 years ago.

doc/filters.texi

@@ -3,13 +3,6 @@
 
 Filtering in FFmpeg is enabled through the libavfilter library.
 
-Libavfilter is the filtering API of FFmpeg. It is the substitute of
-the now deprecated 'vhooks' and started as a Google Summer of Code
-project.
-
-Audio filtering integration into the main FFmpeg repository is a work in
-progress, so audio API and ABI should not be considered stable yet.
-
 In libavfilter, it is possible for filters to have multiple inputs and
 multiple outputs.
 To illustrate the sorts of things that are possible, we can