scripts/kernel_bump: Delete merge commit

While we have included the needed changes via a merge commit, there is
no need to keep it. Lets drop the merge commit, which we can do as we
haven't pushed anything.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>

scripts/kernel_bump.sh

@@ -171,6 +171,8 @@ bump_kernel()
 	git switch "${initial_branch:?Unable to switch back to original branch. Quitting.}"
 	GIT_EDITOR=true git merge --no-ff '__openwrt_kernel_files_mover'
 	git branch --delete '__openwrt_kernel_files_mover'
+	echo "Deleting merge commit ($(git rev-parse HEAD))."
+	git rebase HEAD~1
 
 	echo "Original commitish was '${initial_commitish}'."
 	echo 'Kernel bump complete. Remember to use `git log --follow`.'