remove tests for deleted side panel pinning

https://chromium-review.googlesource.com/c/chromium/src/+/5596995
removed kSidePanelPinning feature,
but it is still used in unittests under
BUILDFLAG(GOOGLE_CHROME_BRANDING) && !BUILDFLAG(IS_CHROMEOS)

e.g.
https://ci.chromium.org/ui/p/chrome/builders/try/linux-chrome-rel-cog-cache-warmer/2596/overview
https://ci.chromium.org/ui/p/chrome/builders/try/linux-chrome-dbg-cog-cache-warmer/1536/overview

Change-Id: I898c22a6e45cde92f4ca211b25caa93f5c90ddad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5606863


Reviewed-by: Philipp Wollermann <philwo@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1311698}