chore: fix linting errors (#5617)

As a result of c09e8a7, a few linting errors got introduced.

Author: crisbeto

src/lib/datepicker/datepicker.spec.ts

@@ -7,7 +7,7 @@ import {MdDatepickerModule} from './index';
 import {MdDatepicker} from './datepicker';
 import {MdDatepickerInput} from './datepicker-input';
 import {MdInputModule} from '../input/index';
-import {MdNativeDateModule, DateAdapter, NativeDateAdapter} from '../core/datetime/index';
+import {MdNativeDateModule} from '../core/datetime/index';
 import {ESCAPE, OverlayContainer} from '../core';
 import {dispatchFakeEvent, dispatchMouseEvent, dispatchKeyboardEvent} from '@angular/cdk/testing';
 import {DEC, JAN} from '../core/testing/month-constants';

src/lib/datepicker/month-view.spec.ts

@@ -3,7 +3,7 @@ import {Component} from '@angular/core';
 import {By} from '@angular/platform-browser';
 import {MdMonthView} from './month-view';
 import {MdCalendarBody} from './calendar-body';
-import {MdNativeDateModule, DateAdapter, NativeDateAdapter} from '../core/datetime/index';
+import {MdNativeDateModule} from '../core/datetime/index';
 import {JAN, MAR} from '../core/testing/month-constants';
 
 describe('MdMonthView', () => {

src/lib/datepicker/year-view.spec.ts

@@ -3,7 +3,7 @@ import {Component} from '@angular/core';
 import {By} from '@angular/platform-browser';
 import {MdYearView} from './year-view';
 import {MdCalendarBody} from './calendar-body';
-import {MdNativeDateModule, DateAdapter, NativeDateAdapter} from '../core/datetime/index';
+import {MdNativeDateModule} from '../core/datetime/index';
 import {FEB, JAN, MAR} from '../core/testing/month-constants';
 
 describe('MdYearView', () => {