board: am335x: Drop duplicate pinmux configuration

board: am335x: Drop duplicate pinmux configuration

In commit ad6054f1fe128f797b6eb2964afca6674b584785 where support for the
Sancloud BeagleBone Enhanced (BBE) was added, new conditional
configuration of either MII pin muxing or RGMII pin muxing is done
depending on the board type. However, the old call to set up MII pin
muxing was not removed.

This may result in misconfiguration of the pin muxing for the BBE or
duplicate configuration for other boards and so we remove this obsolete
call.

Signed-off-by: Paul Barker <paul.barker@sancloud.co.uk>