🧹 Remove override of thumbnail service

[jeremyf]

This is now obviated by upstream Hyku. Prior to this commit, we were looking in Rails.root but we needed to first interogate the Knapsack and then Rails.root.

Related to:

• https://github.com/samvera/hyku/issues/2010
• https://github.com/samvera/hyku/pull/2012
• https://github.com/scientist-softserv/adventist_knapsack/issues/60
• https://github.com/samvera-labs/hyku_knapsack/issues/1