refactor: Remove unimportant labels

Removed "label" properties which no longer have any function.

Labels are still used as layer names and as identifiers for sending
behaviors between sides of a split keyboard, so those have been left
alone for now.

app/boards/native_posix.overlay

@@ -9,7 +9,6 @@
 
     kscan: kscan {
         compatible = "zmk,kscan-mock";
-        label = "KSCAN_MOCK";
 
         rows = <2>;
         columns = <2>;