mpv: Remove waf script from buildInputs.

Since b23dbb1a5dffbfa3abb47fcd0f1579ac2e6f29fc, if buildInputs contains
a plain file it is used as a setup hook. The waf script which is used
here in mpv however isn't a setup hook and also shouldn't be included in
buildInputs as it was kind of a no-op before already.

Failed build log:

https://headcounter.org/hydra/build/582548/nixlog/1/raw

Signed-off-by: aszlig <aszlig@redmoonstudios.org>