boards: remove app support for custom_plank and nrf54l15

Refs: #5

Signed-off-by: Natalia Pluta <pluta.natalia.m@gmail.com>

Author: nmpluta

README.md

@@ -46,9 +46,6 @@ west build -b $BOARD app
 
 where `$BOARD` is the target board.
 
-You can use the `custom_plank` board found in this repository. Note that you can use
-Zephyr and nRF Connect SDK sample boards if an appropriate overlay is provided (see `app/boards`).
-
 A sample debug configuration is also provided. To apply it, run the following
 command:
 

app/sample.yaml

@@ -8,8 +8,6 @@ common:
   sysbuild: true
   build_only: true
   integration_platforms:
-    - custom_plank
-    - nrf54l15dk/nrf54l15/cpuapp
     - nrf9160dk/nrf9160
 tests:
   app.default: {}