Introduced in 1992, the Alpha was a 64-bit RISC processor designed
to replace the VAX CISC machines sold by Digital Equipment Corporation.

After Digital was acquired by Compaq in 1998 -- who themselves would be
later purchased by Hewlett-Packard, the architecture was phased out over
the following decade. It became effectively defunct in 2007, the last
publicly available processor being the Alpha 21364.

FFmpeg has not added any DSP code for this architecture since lowres2
was introduced in 2012, and it is more than unlikely someone still wishes
to maintain it.

Remove the DSP and support code.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>