switchdev: Complete removal of switchdev_port_attr_get()

We have no more in tree users of switchdev_port_attr_get() after
d0e698d5

 ("Merge branch 'net-Get-rid-of-switchdev_port_attr_get'")
so completely remove the function signature and body.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>