Refactor project_templates spec

What does this MR do and why?

Describe in detail what your merge request does and why.

Replaces before(:all) and instance variables with let as shared setup prevents individually quarantined examples from running correctly (see #364742 (comment 993384822) and https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/7367056). Also removes unnecessary project cleanup as we now have test resources cleanup mechanism in place and it can be highly beneficial for debugging to keep some resources for investigation after a failure.

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