refactor(portal): remove 6.0.0 deletion targets (#10257)

Removes the deletion targets for 6.0.0 in the `cdk/portal` entry point.

BREAKING CHANGES:
* The `portalHost` and `cdkPortalHost` inputs which were deprecated in 5.0.0 have been removed. Use `cdkPortalOutlet` instead.

Author: crisbeto

src/cdk/portal/portal-directives.ts

@@ -16,7 +16,6 @@ import {
   ViewContainerRef,
   OnDestroy,
   OnInit,
-  Input,
   EventEmitter,
   Output,
 } from '@angular/core';
@@ -68,22 +67,6 @@ export class CdkPortalOutlet extends BasePortalOutlet implements OnInit, OnDestr
     super();
   }
 
-  /**
-   * @deprecated
-   * @deletion-target 6.0.0
-   */
-  @Input('portalHost')
-  get _deprecatedPortal() { return this.portal; }
-  set _deprecatedPortal(v) { this.portal = v; }
-
-  /**
-   * @deprecated
-   * @deletion-target 6.0.0
-   */
-  @Input('cdkPortalHost')
-  get _deprecatedPortalHost() { return this.portal; }
-  set _deprecatedPortalHost(v) { this.portal = v; }
-
   /** Portal associated with the Portal outlet. */
   get portal(): Portal<any> | null {
     return this._attachedPortal;