Cleanup all browser layer properties dialog before exiting

[nyalldawson]

These must be deleted before calling QgsApplication::exitQgis, or we get a crash

Fixes #39094

Description

[Replace this with some text explaining the rationale and details about this pull request]

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 60f04e93df... Cleanup all browser layer properties dialog before exiting
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

Auto-merging src/app/qgisapp.cpp
CONFLICT (content): Merge conflict in src/app/qgisapp.cpp

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-54808-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 60f04e93df628dc950f19ec2d7091bae1d6d4178
# Push it to GitHub
git push --set-upstream origin backport-54808-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-54808-to-queued_ltr_backports.