*/*: De-orbit alias.mk

Mk/Uses/alias.mk was an attempt for compat with DPorts, but has rotted.
This feature was never documented in the PHB, so there is no work to
be done by the docs@ team.

Nothing against DragonFly, but if DPorts still needs FreeBSD 9
support, they really should work that out with upstream projects.

Ultimately, we don't need to keep carrying this outdated feature that
doesn't affect us.