avcodec: Remove libaacplus

TODO: bump minor

It's inferior in quality to fdk-aac and has an arguably more problematic
license.

As early as 2012, a HydrogenAudio user reported:

> It has however one huge advantage: much better quality at low bitrates than
> faac and libaacplus.

(https://hydrogenaud.io/index.php?PHPSESSID=ckiq394pdglka0kj2fin6ij8t7&topic=95989.msg804633#msg804633)

I myself have made a few spectrograms for a comparison of the two
encoders as well. The FDK output is consistently better than the
libaacplus one, in all bitrates I tested.

libaacplus license is 3GPP + LGPLv2. 3GPP copyright notice is completely
proprietory, as follows:

> No part may be reproduced except as authorized by written permission.
>
> The copyright and the foregoing restriction extend to reproduction in
> all media.
>
> © 2008, 3GPP Organizational Partners (ARIB, ATIS, CCSA, ETSI, TTA, TTC).
>
> All rights reserved.

(The latest 26410-d00 zip from 3GPP has the same notice, but the copyright
year is changed to 2015)

The copyright part of the FDK AAC license (section 2) is a copyleft
license that permits redistribution under certain conditions (and
therefore the LGPL + libfdk-aac combination is not prohibited by
configure):

> Redistribution and use in source and binary forms, with or without
> modification, are permitted without payment of copyright license fees
> provided that you satisfy the following conditions:
>
> You must retain the complete text of this software license in
> redistributions of the FDK AAC Codec or your modifications thereto in
> source code form.
>
> You must retain the complete text of this software license in the
> documentation and/or other materials provided with redistributions of
> the FDK AAC Codec or your modifications thereto in binary form.
>
> You must make available free of charge copies of the complete source
> code of the FDK AAC Codec and your modifications thereto to recipients
> of copies in binary form.
>
> The name of Fraunhofer may not be used to endorse or promote products
> derived from this library without prior written permission.
>
> You may not charge copyright license fees for anyone to use, copy or
> distribute the FDK AAC Codec software or your modifications thereto.
>
> Your modified versions of the FDK AAC Codec must carry prominent
> notices stating that you changed the software and the date of any
> change. For modified versions of the FDK AAC Codec, the term
> "Fraunhofer FDK AAC Codec Library for Android" must be replaced by the
> term "Third-Party Modified Version of the Fraunhofer FDK AAC Codec
> Library for Android."

doc/platform.texi

@@ -314,7 +314,7 @@ These library packages are only available from
 @uref{http://sourceware.org/cygwinports/, Cygwin Ports}:
 
 @example
-yasm, libSDL-devel, libfaac-devel, libaacplus-devel, libgsm-devel, libmp3lame-devel,
+yasm, libSDL-devel, libfaac-devel, libgsm-devel, libmp3lame-devel,
 libschroedinger1.0-devel, speex-devel, libtheora-devel, libxvidcore-devel
 @end example