soc: nordic: nrf54h: Remove external square wave

This option is no longer present in the Datasheet, remove it from the
BICR JSON file.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

Author: carlescufi

soc/nordic/nrf54h/bicr/bicr-schema.json

@@ -350,12 +350,10 @@
           "type": "string",
           "title": "Mode",
           "enumNames": [
-            "Crystal",
-            "External square signal"
+            "Crystal"
           ],
           "enum": [
-            "CRYSTAL",
-            "EXT_SQUARE"
+            "CRYSTAL"
           ],
           "default": "CRYSTAL"
         },