ASoC: jz4740-i2s: Remove .set_sysclk() & friends

Merge series from Aidan MacDonald <aidanmacdonald.0x0@gmail.com>:

A quick series to get rid of .set_sysclk() from jz4740-i2s.
It wasn't used in practice so this shouldn't be troublesome for anyone,
and fortunately there aren't any backward compatibility concerns.

The actual rationale for removing it, as opposed to fixing the
issues of the current DT bindings and implementation, is provided
in the dt-bindings patch.