(Maint) Remove fragile test

This test began to fail with

1. Puppet::SSL::Key should default to the :file terminus Failure/Error: @class.indirection.terminus_class.should == :file expected: :file got: :disabled_ca (using ==) Diff: @@ -1,2 +1,2 @@ -:file +:disabled_ca
   1. ./spec/unit/ssl/key_spec.rb:20

which was caused by other tests setting the terminus_class for the
Puppet::SSL::Key class. Since this is testing no interesting logic and
just looking at a default (that can be changed and not recovered), I'm
just removing the test.