Remove unused saas tag from non-EE specs

What does this MR do and why?

The tag :saas is currently defined in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/support/stub_dot_com_check.rb and thus not available when run with FOSS_ONLY=1.

Note that the specs are still passing so the tag is probably unused.

Refs #367039 (comment 1024930881)

How to set up and validate locally

FOSS_ONLY=1 bin/rspec spec/features/groups_spec.rb spec/helpers/storage_helper_spec.rb spec/lib/gitlab/ci/pipeline/chain/validate/external_spec.rb

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