samples: net: lwm2m: cleanup prj.conf

- Remove NET_LOG / NET_BUF_LOG settings, instead just enable SYS_LOG
- Simplify by also removing SYS_LOG_SHOW_COLOR, INIT_STACKS and
  NET_STATISTICS.  These can be enabled by user if wanted/needed.

Signed-off-by: Michael Scott <mike@foundries.io>