chore: Remove PSS-related gating flag/ENV in the init command

[SarahFrench]

Follows #38227

That previous PR combines the two versions of init logic that were present when PSS was in earlier development (see PR description). The choice of which version to use was controlled by a flag -enable-pluggable-state-storage-experiment, and that PR removed the ability to make that choice; all users now experience the new init flow that is necessary for PSS.

It should now be that anyone can use PSS as long as experiments are enabled (and they have a provider implementing it). However that alpha still returns this error that enforces the flag being provided:

│ Error: Pluggable state store experiment not supported
│
│ on main.tf line 7, in terraform:
│ 7: state_store "pss_fs" {
│
│ Pluggable state store is an experiment which requires -enable-pluggable-state-storage-experiment flag

This PR removes remaining code related to the flag -enable-pluggable-state-storage-experiment and the related ENV TF_ENABLE_PLUGGABLE_STATE_STORAGE.

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

It is technically user-facing, but this is all still gated behind experiments being enabled, so not user facing by the definition that matters here.

[mildwonkey]

This is just a note for future code writing, no changes needed, but for what it's worth, c.Run(nil) or c.Run([]string{}) also work fine, and don't allocate an otherwise unused variable. I try to avoid variables that aren't adding anything but lines (again, not hurting anything, no changes needed).

[SarahFrench]

We've decided to pause removing this until GA, so I'm gonna save merging this until the next beta is out

[radeksimko]

Marking this as draft for now until we are ready to remove the gating, as per our Slack conversation.