Removed unused modal-custom-view-form.js file

no issue

- the `custom-view-form` modal has been migrated to the newer modal style but the original modal's component file wasn't cleaned up

ghost/admin/app/components/modal-custom-view-form.js

@@ -1,58 +0,0 @@
-import ModalComponent from 'ghost-admin/components/modal-base';
-import classic from 'ember-classic-decorator';
-import {action} from '@ember/object';
-import {resetQueryParams} from 'ghost-admin/helpers/reset-query-params';
-import {inject as service} from '@ember/service';
-import {task, timeout} from 'ember-concurrency';
-
-@classic
-export default class ModalCustomViewForm extends ModalComponent {
-    @service customViews;
-    @service router;
-
-    delayedModelColor = '';
-
-    init() {
-        super.init(...arguments);
-        this.set('model', this.customViews.editView());
-        this._setDelayedModelColor.perform();
-    }
-
-    @action
-    changeColor(event) {
-        let color = event.target.value;
-        this.set('model.color', color);
-        this.set('delayedModelColor', color);
-    }
-
-    @action
-    confirm() {
-        return this.saveTask.perform();
-    }
-
-    @task(function* () {
-        let view = yield this.customViews.saveViewTask.perform(this.model);
-        this.send('closeModal');
-        return view;
-    })
-        saveTask;
-
-    @task(function* () {
-        let view = yield this.customViews.deleteViewTask.perform(this.model);
-        let routeName = this.router.currentRouteName;
-        this.send('closeModal');
-        this.router.transitionTo(routeName, {queryParams: resetQueryParams(routeName)});
-        return view;
-    })
-        deleteTask;
-
-    // this is a hack to get around radio buttons not working with liquid-fire.
-    // The DOM is duplicated whilst animating-in so browsers end up setting the
-    // checked property on the temporary DOM. Delaying the value being set
-    // allows us to ensure we're updating the checked property after animation
-    @task(function* () {
-        yield timeout(200);
-        this.set('delayedModelColor', this.model.color);
-    })
-        _setDelayedModelColor;
-}