promgen.vue.js: Fix silence form's delete label

Before creating the silence, the silence form appears and we have the
possibility to remove any labels that were added by default. When doing
so, an error like this one happens:

  TypeError: this.$delete is not a function

The error occurs because the "$delete" instance method [1] has been
removed in Vue 3 [2], but we forgot to adapt this part of the code when
we did the migration.

1: https://v2.vuejs.org/v2/api/#vm-delete
2: https://v3-migration.vuejs.org/breaking-changes/#removed-apis

promgen/static/js/promgen.vue.js

@@ -158,7 +158,7 @@ app.component('silence-form', {
     }),
     methods: {
         removeLabel(label) {
-            this.$delete(this.state.labels, label);
+            delete this.state.labels[label];
         },
         submit() {
             const body = JSON.stringify({