doc/encoders: remove unsubstantiated ffaacenc > fdk-aac claim

After this claim was made in e34e361 kamedo2 did an in-depth ABX
test comparing these encoders:

https://hydrogenaud.io/index.php?topic=111085.0

Result: FFmpeg AAC wasn't as good as libfdk_aac on average.

I know some things have changed since then such as, "use the fast
coder as the default" (fcb681ac) for example, so maybe the situation
is different now.

However, I am unaware of any recent comparison. So without any
substantiation we shouldn't make such a blantant claim.

Signed-off-by: Lou Logan <lou@lrcd.com>
Signed-off-by: Gyan Doshi <ffmpeg@gyani.pro>

doc/encoders.texi

@@ -30,11 +30,7 @@ follows.
 
 Advanced Audio Coding (AAC) encoder.
 
-This encoder is the default AAC encoder, natively implemented into FFmpeg. Its
+This encoder is the default AAC encoder, natively implemented into FFmpeg.
-quality is on par or better than libfdk_aac at the default bitrate of 128kbps.
-This encoder also implements more options, profiles and samplerates than
-other encoders (with only the AAC-HE profile pending to be implemented) so this
-encoder has become the default and is the recommended choice.
 
 @subsection Options
 
@@ -651,10 +647,7 @@ configuration. You need to explicitly configure the build with
 so if you allow the use of GPL, you should configure with
 @code{--enable-gpl --enable-nonfree --enable-libfdk-aac}.
 
-This encoder is considered to produce output on par or worse at 128kbps to the
+This encoder has support for the AAC-HE profiles.
-@ref{aacenc,,the native FFmpeg AAC encoder} but can often produce better
-sounding audio at identical or lower bitrates and has support for the
-AAC-HE profiles.
 
 VBR encoding, enabled through the @option{vbr} or @option{flags
 +qscale} options, is experimental and only works with some