Remove persistent_gems_after_clean workaround

We put this check in place as a workaround for an internal project at
GitHub that was using some custom extensions to bundler. This has since
been removed, and this check is no longer relevant, so let's clean it
up.

bundler/helpers/v2/lib/functions/lockfile_updater.rb

@@ -65,7 +65,7 @@ def cache_vendored_gems(definition)
       # Dependencies that have been unlocked for the update (including
       # sub-dependencies)
       unlocked_gems = definition.instance_variable_get(:@unlock).
-                      fetch(:gems).reject { |gem| __keep_on_prune?(gem) }
+                      fetch(:gems)
       bundler_opts = {
         cache_all: true,
         cache_all_platforms: true,
@@ -85,15 +85,6 @@ def cache_vendored_gems(definition)
       end
     end
 
-    # This is not officially supported and may be removed without notice.
-    def __keep_on_prune?(spec_name)
-      unless (specs = Bundler.settings[:persistent_gems_after_clean])
-        return false
-      end
-
-      specs.include?(spec_name)
-    end
-
     # Copied from Bundler::Runtime: Modified to only prune gems that have
     # been unlocked
     def prune_gem_cache(resolve, cache_path, unlocked_gems)

bundler/spec/dependabot/bundler/file_updater_spec.rb

@@ -1495,22 +1495,6 @@
         expect(file.operation).to eq Dependabot::DependencyFile::Operation::DELETE
       end
 
-      context "persistent gems after clean" do
-        let(:project_name) { "vendored_persistent_gems" }
-
-        it "does not delete cached files marked as persistent" do
-          file = updater.updated_dependency_files.find do |f|
-            f.name == "vendor/cache/business-1.4.0.gem"
-          end
-
-          vendor_files =
-            Dir.entries(Pathname.new(repo_contents_path).join("vendor/cache"))
-
-          expect(file).to be_nil
-          expect(vendor_files).to include("business-1.4.0.gem")
-        end
-      end
-
       context "with dependencies that are not unlocked by the update" do
         let(:project_name) { "conditional" }