Removing random sampling APIs

[masseyke]

This removes the APIs that were added in #5528 and #5573. They were behind a feature flag, and have since been deleted (in elastic/elasticsearch#143289).

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

API	Status	Request	Response
indices.delete_sample_configuration	➖ ⚪	Missing test	Missing test
indices.get_all_sample_configuration	➖ ⚪	Missing test	Missing test
indices.get_sample	➖ ⚪	Missing test	Missing test
indices.get_sample_configuration	➖ ⚪	Missing test	Missing test
indices.get_sample_stats	➖ ⚪	Missing test	Missing test
indices.put_sample_configuration	➖ ⚪	Missing test	Missing test

You can validate these APIs yourself by using the make validate target.

[seanzatzdev]

LGTM

[github-actions]

The backport to 9.3 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.3 9.3
# Navigate to the new working tree
cd .worktrees/backport-9.3
# Create a new branch
git switch --create backport-6089-to-9.3
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 8e73926ea22d155a2ef2f65cdb45f0fe9ad082f2
# Push it to GitHub
git push --set-upstream origin backport-6089-to-9.3
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.3

Then, create a pull request where the base branch is 9.3 and the compare/head branch is backport-6089-to-9.3.