Remove outdated code which prevents setting activity task in some scenarios

It is safe to remove because the original NPE reported in b/70635977
was fixed after f7f7f9c4.

Bug: 139449647
Test: atest ActivityStarterTests#testClearTopNewTaskResetTask
Change-Id: I6af00938d9258722a10da07accdc329f28134f50
1 file changed