DEV: prevents flakky spec when deleting plugin (PR #14701)

Not reseting the registry could lead to assets still being registered for example.

This flakky spec was reprdocible with this call: bundle exec rspec --seed 9472 spec/components/discourse_plugin_registry_spec.rb spec/components/svg_sprite/svg_sprite_spec.rb

Which would trigger the following error:

Failures:

  1) DiscoursePluginRegistry#register_asset registers vendored_core_pretty_text properly
     Failure/Error: expect(registry.javascripts.count).to eq(0)

       expected: 0
            got: 1

       (compared using ==)
     # ./spec/components/discourse_plugin_registry_spec.rb:248:in `block (3 levels) in <top (required)>'
     # ./spec/rails_helper.rb:280:in `block (2 levels) in <top (required)>'
     # /Users/joffreyjaffeux/.gem/ruby/2.7.3/gems/webmock-3.14.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'

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