Removed unused fixtures.

spec/fixtures/emails/boundary.eml

spec/fixtures/emails/multiple_destinations.eml

spec/fixtures/emails/paragraphs.cooked

spec/fixtures/emails/previous.eml

spec/fixtures/emails/previous_replies.eml

spec/fixtures/emails/too_many_mentions.eml

spec/fixtures/emails/valid_incoming.cooked

spec/fixtures/emails/valid_incoming.eml

spec/fixtures/emails/valid_reply.cooked

spec/fixtures/emails/via_line.eml

spec/lib/gitlab/email/receiver_spec.rb