b43: remove leftover code from old devices support

Old devices (A-PHY or B-PHY) are supposed to be supported by b43legacy.
We keep phy_a.c as it's needed for G-PHY which shares some design.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>