Remove FLATPAK_ENABLE_EXPERIMENTAL_OCI envvar check

Now that appstream data and icons are retrievded from the index, the OCI
code is expected to be fully usable, and to work with registries as they
will actually be deployed.

Closes: #1910
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f