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/modifiers/implicit/kp-mod1-dn-mod2-dn-mod1-up-mod2-up/native_posix_64.keymap

@@ -15,7 +15,6 @@
 / {
     keymap {
         compatible = "zmk,keymap";
-        label ="Default keymap";
 
         default_layer {
             bindings = <

app/tests/modifiers/implicit/kp-mod1-dn-mod2-dn-mod2-up-mod1-up/native_posix_64.keymap

@@ -15,7 +15,6 @@
 / {
     keymap {
         compatible = "zmk,keymap";
-        label ="Default keymap";
 
         default_layer {
             bindings = <

app/tests/modifiers/implicit/kp-rolling-symbols-same-key/native_posix_64.keymap

@@ -15,7 +15,6 @@
 / {
     keymap {
         compatible = "zmk,keymap";
-        label ="Default keymap";
 
         default_layer {
             bindings = <