Delete orphaned test files already moved to subpackages

These 7 test files in test/interface/ were copied to
OrdinaryDiffEqDifferentiation and OrdinaryDiffEqNonlinearSolve
subpackages in PR SciML#3156 but the originals were never removed.
They are no longer referenced by test/runtests.jl.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas