doc: board_porting: Fix Kconfig.defconfig example

The part that says "Always set CONFIG_BOARD here" is outdated. In HWMv2,
CONFIG_BOARD is set by the build system, and we don't want to encourage
users to set it manually.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>

doc/hardware/porting/board_porting.rst

@@ -571,13 +571,6 @@ files for a board named ``plank``:
 
  if BOARD_PLANK
 
- # Always set CONFIG_BOARD here. This isn't meant to be customized,
- # but is set as a "default" due to Kconfig language restrictions.
- config BOARD
- default "plank"
-
- # Other options you want enabled by default go next. Examples:
-
  config FOO
  default y