rspec: Remove Puppet::Util::Storage mock

In attempting to upgrade bodgit/puppet-postfix's[^1] dependency on this
module, I received the following error:

  Failure/Error: allow(Puppet::Util::Storage).to receive(:store)
    The use of doubles or partial doubles from rspec-mocks outside
    of the per-test lifecycle is not supported.

This appears to be the result of the change in
637108e to use a mock rather than a
stub. However, it is not clear to me why Puppet::Util::Storage needs to
be mocked, as all tests still pass in this module and puppet-postfix
without it, so remove.

[1]: https://github.com/bodgit/puppet-postfix

spec/lib/augeas_spec.rb

@@ -13,8 +13,4 @@ class AugeasSpec::Error < StandardError
   config.extend AugeasSpec::Fixtures
   config.include AugeasSpec::Augparse
   config.include AugeasSpec::Fixtures
-
-  config.before do
-    allow(Puppet::Util::Storage).to receive(:store)
-  end
 end