Remove irrelevant failing tests

Those were testing the isignatures of nherited methods, whereas these were actually proxified
to a delegate object that could have complete different signatures without the test to notice.
Now rely on the other test verifying that methods are proxified to the delegate object,
transmitting the expected arguments.

Author: bgandon

src/bosh-director/spec/unit/blobstore_client/retryable_blobstore_client_spec.rb

@@ -6,7 +6,6 @@ module Bosh::Blobstore
     let(:wrapped_client) { instance_double('Bosh::Blobstore::BaseClient') }
     let(:retryable)      { Bosh::Retryable.new(tries: 2, sleep: 0, on: [BlobstoreError]) }
 
-    it_implements_base_client_interface
     it_calls_wrapped_client_methods(except: [:get])
 
     describe '#get' do

src/bosh-director/spec/unit/blobstore_client/sha1_verifiable_blobstore_client_spec.rb

@@ -6,7 +6,6 @@ module Bosh::Blobstore
     let(:wrapped_client) { instance_double('Bosh::Blobstore::BaseClient') }
     let(:multidigest_path) { 'some/path/to/binary' }
 
-    it_implements_base_client_interface
     it_calls_wrapped_client_methods(except: [:get])
 
     before do