perf(module:date-picker): remove unused variable (#7767)

Author: HyperLife1119

components/date-picker/date-picker.component.ts

@@ -48,7 +48,7 @@ import { slideMotion } from 'ng-zorro-antd/core/animation';
 import { NzConfigKey, NzConfigService, WithConfig } from 'ng-zorro-antd/core/config';
 import { NzFormNoStatusService, NzFormStatusService } from 'ng-zorro-antd/core/form';
 import { NzNoAnimationDirective } from 'ng-zorro-antd/core/no-animation';
-import { DEFAULT_DATE_PICKER_POSITIONS, DATE_PICKER_POSITION_MAP } from 'ng-zorro-antd/core/overlay';
+import { DATE_PICKER_POSITION_MAP, DEFAULT_DATE_PICKER_POSITIONS } from 'ng-zorro-antd/core/overlay';
 import { CandyDate, cloneDate, CompatibleValue, wrongSortOrder } from 'ng-zorro-antd/core/time';
 import {
   BooleanInput,
@@ -632,7 +632,7 @@ export class NzDatePickerComponent implements OnInit, OnChanges, OnDestroy, Afte
     // Default value
     this.datePickerService.isRange = this.isRange;
     this.datePickerService.initValue(true);
-    this.datePickerService.emitValue$.pipe(takeUntil(this.destroyed$)).subscribe(_ => {
+    this.datePickerService.emitValue$.pipe(takeUntil(this.destroyed$)).subscribe(() => {
       const value = this.datePickerService.value;
       this.datePickerService.initialValue = cloneDate(value);
       if (this.isRange) {