[file manager] Remove drop effect label code

After https://crrev.com/c/2899860 label for drop effect is no more used,
but is still selected and stored. This change cleans it up.

Bug: b:276413049
Change-Id: I3618df37b321fc0970682002482a582a3174a30d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4552081


Auto-Submit: Oleg Davydov <burunduk@chromium.org>
Reviewed-by: Austin Tankiang <austinct@chromium.org>
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148943}