app_list: Clean up apps_grid_view_unitest.cc, part 5

Clean up tests that read `is_productivity_launcher_enabled_` where that
boolean is always true.

Delete the test ControlArrowDownAndRightCreatesNewPage. When
productivity launcher is disabled, new pages can't be created with the
keyboard. When the flag is false, this test basically just asserts
that nothing happens, so delete the test.

Bug: 1360501
Test: ash_unittests
Change-Id: Ic69ddf391468ead307e900b0dda1d6100ccad7ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3900460
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Wen-Chien Wang <wcwang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1047729}