Remove stale TEST_PLAN.md and drop docs/ from gemspec files

TEST_PLAN.md was the pre-implementation test plan written in
red-phase-first mode. It still referenced the Discovery class and
clock_skew option that were deleted in the recent review pass, and
its utility has been entirely superseded by the 104 passing specs,
which are the living documentation of the gem's behaviour.

Also drops 'docs/**/*' from the gemspec files list — internal dev
docs have no business shipping inside the installed gem payload.
Nothing in docs/ was user-facing anyway.

Author: Fivell

activeadmin-oidc.gemspec

@@ -22,7 +22,6 @@ Gem::Specification.new do |spec|
     "lib/**/*",
     "app/**/*",
     "config/**/*",
-    "docs/**/*",
     "README.md",
     "LICENSE.txt"
   ]