Remove the repl-flake experimental feature

[thufschmitt]

Is your feature request related to a problem? Please describe.

The repl-flake experimental feature was adopted to provide a migration path when changing the behaviour of nix repl (#6233 (review)), with the intent of removing it after a few releases.
More than a few releases have passed, now is time to rip off the band-aid.

(There's one issue related to its semantics, but that's specific to flakes, so it's not a blocker here)

Priorities

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[thufschmitt]

#9868 seems to be a blocker actually

[detroyejr]

Looks like the issue blocking this has been resolved.

Could you confirm that a PR of this scope is what's needed?

1. Remove any entries for repl-flakes.
2. Remove applyDefaultNullables
3. Fix tests in repl.sh if 2 applies.

nix repl moving foward would behave as follows:

# No longer allowed and returns the same error as nix run.
nix repl default.nix
> error: path 'default.nix' is not a flake (because it's not a directory)

nix repl --file default.nix
> Loading installable ''...
> Added 13 variables.

# Still require experimental features flag flakes.
nix repl .
> error: experimental Nix feature 'flakes' is disabled ...

# Works as before.
nix repl --extra-experimental-features flakes .

[thufschmitt]

Could you confirm that a PR of this scope is what's needed?

1. Remove any [entries](https://github.com/NixOS/nix/blob/a22328066416650471c3545b0b138669ea212ab4/src/libutil/experimental-features.cc#L206) for repl-flakes.

2. Remove [applyDefaultNullables](https://github.com/NixOS/nix/blob/a22328066416650471c3545b0b138669ea212ab4/src/nix/repl.cc#L48)

3. Fix tests in repl.sh if 2 applies.

I confirm. And I can also say that I would be very grateful if you're willing to take care of this :)

[thufschmitt]

Oh, there's just one extra step:

4. Add a release note under doc/manual/rl-next