net: phylink: remove "using_mac_select_pcs"

With DSA's implementation of the mac_select_pcs() method removed, we
can now remove the detection of mac_select_pcs() implementation.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>