realtek: pcs: drop unused cmu_band code

realtek: pcs: drop unused cmu_band code

For RTL930x and RTL931x, there were some functions to get and set the
CMU band. However, they were used nearly nowhere and even the SDK
doesn't use them for active SerDes configuration. Drop them to reduce
dead code.

Dropping the cmu_band_set call from RTL931x configuration has no
negative effect. We do not know where this call originally came from.
It was introduced 51c8f76612 ("realtek: improve MAC config handling for
all SoCs") but without any explanation why and where it came from. The
SDK doesn't have it and it may be overwritten again in the later CMU
setup when parts of the SerDes are reset.

Link: https://github.com/openwrt/openwrt/pull/23904
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>