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/tests/mod-morph/2b-masked-morph-implicit-overwrite/native_posix_64.keymap

@@ -6,7 +6,6 @@
     behaviors {
         mod_morph: mod_morph {
             compatible = "zmk,behavior-mod-morph";
-            label = "MOD_MORPH_TEST";
             #binding-cells = <0>;
             bindings = <&kp A>, <&kp LS(B)>;  // implict mod overwrite
             mods = <(MOD_LSFT|MOD_RSFT)>;
@@ -15,7 +14,6 @@
 
     keymap {
         compatible = "zmk,keymap";
-        label ="Default keymap";
 
         default_layer {
             bindings = <