librem5-evk: Remove superfluous clock nodes

The sound_osc and mclk nodes aren't actually used by anything.
Removing them firstly simplies the .dtsi and secondly avoids any
potential confusion.
2 jobs for clocks-cleanup in 43 minutes and 33 seconds
latest
Status Job ID Name Coverage
  Build
passed #10664
librem5
build:aarch64:Image

00:43:33

passed #10665
librem5
build:aarch64:deb

00:28:25