GTiff: fix to not delete DIMAP XML files when cleaning overviews on a DIMAP2 GeoTIFF file with external overviews

[coveralls]

coverage: 69.418% (-0.002%) from 69.42% when pulling 664799a95bb2266bd75f9bab103efc5613130d77 on rouault:tiff_do_not_delete_dim_on_ovr into 6aa2e2e2ab89a35d71d4cb87d1fd3f4a8be5fadd on OSGeo:master.

[rouault]

The backport to release/3.9 failed:

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

stderr

``` error: could not apply 664799a95b... GTiff: fix to not delete DIMAP XML files when cleaning overviews on a DIMAP2 GeoTIFF file with external overviews hint: After resolving the conflicts, mark them with hint: "git add/rm ", 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". hint: Disable this message with "git config advice.mergeConflict false" ```

stdout

``` Auto-merging autotest/gcore/tiff_read.py CONFLICT (content): Merge conflict in autotest/gcore/tiff_read.py Auto-merging frmts/gtiff/gtiffdataset_read.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-release/3.9 release/3.9
# Navigate to the new working tree
cd .worktrees/backport-release/3.9
# Create a new branch
git switch --create backport-10868-to-release/3.9
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 664799a95bb2266bd75f9bab103efc5613130d77
# Push it to GitHub
git push --set-upstream origin backport-10868-to-release/3.9
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.9

Then, create a pull request where the base branch is release/3.9 and the compare/head branch is backport-10868-to-release/3.9.

[rouault]

cherry-picked in 3.9 per cb1b652f59531aa599517a772b08f5178e1a4383