aacenc: remove support for AAC LTP profile

The LTP profile of AAC is... terrible.
It was an early 90's attempt at bridging the gap between speech
codecs and general purpose codecs. It did so by trying to exploit the fact
that most speech patterns are regular.

Unfortunately, it went about it the same way as AAC Main, by taking
the previous frame's samples, modifying them through an LPC filter,
transforming them back using a forward MDCT, putting the output
coefficients back into the current frame, and using delta coding.
But once again, they ignored basic mathematics and MDCT leakage.
Thankfully, because AAC LTP is meant to operate at very low bitrates,
the extreme quantization results in most leakage being irrelevant.

Unfortunately, the result is that the output sounds pretty much
terrible regardless of whether LTP is enabled or not.

This was the first attempt at trying to couple speech coding into AAC.
No, the second attempt did not succeed either.
Nnnneither did the third. Or fourth.

For the fifth one, they literally just jammed a speech codec into AAC
with USAC once they saw Opus do it.

Just drop support for encoding AAC LTP. It was always experimental
to begin with.

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