Remove usage of deleted manifest_dir

Recent update in rspec-puppet removed some of the config interfaces for
old puppet versions[1]. This drops usage of these interfaces to resolve
the following error in unit tests.

```
An error occurred while loading ./spec/unit/provider/manila_spec.rb.
Failure/Error: c.manifest_dir = File.join(fixture_path, 'manifests')

NoMethodError:
  undefined method `manifest_dir='...
```

This also removes explicit setting of mock module. The definition is
no longer required since we bumped puppetlabs_spec_helper to v 5.0.0.

[1] 316d95923c

Change-Id: Id94f7feeec26e3efdc8e5463deaa52ee44b7d07f

spec/spec_helper.rb

@@ -9,9 +9,6 @@ RSpec.configure do |c|
   c.alias_it_should_behave_like_to :it_raises, 'raises'
 
   c.module_path = File.join(fixture_path, 'modules')
-  c.manifest_dir = File.join(fixture_path, 'manifests')
-
-  c.mock_with :rspec
 end
 
 at_exit { RSpec::Puppet::Coverage.report! }