Remove GITLAB_TEMPFILE_IMMEDIATE_UNLINK env var

Sean McGivernrequested to merge
remove-tempfile-unlink-env-var into master

We have rolled this out in production and seen that it works: no increase in error rate, and temporary files do not hang around any more.

See https://gitlab.com/gitlab-org/gitlab/-/issues/324817 and gitlab-com/gl-infra/production#4212 (closed) for more details.

The final step after this is to propose the Puma change upstream.

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