[Cleanup] Remove deprecated dev/import-beats script

## Summary

The `dev/import-beats` script has been officially marked as **deprecated** since July 2022
(see #3652 / commit `75a9980a`). Its README explicitly states:

> The script is not actively maintained and we (@ecosystem team) would rather deprecate it
> to prevent its accidental future adoption in developer workflows.

It was a one-shot migration aid to convert Beats modules into the initial skeleton of
Elastic Integration packages. All integrations that needed to be bootstrapped from Beats
have long since been migrated. The script has had no functional changes since September
2022 (Go 1.19 version bump), and the deprecation notice has been in place for ~4 years.

## Motivation

- The script is dead code: no active use case, no maintenance, and the ecosystem team
explicitly discourages adoption.
- Keeping it in the repository creates a risk of accidental re-adoption by new contributors
who may not notice or read the deprecation warning.
- Removing it reduces the maintenance surface (Go dependencies, linting, CI build time).

## Proposed Action

Remove the entire `dev/import-beats/` directory and the related `mage ImportBeats` target
from the `Magefile.go` (if still wired up).

## Checklist

- [ ] Remove `dev/import-beats/` directory
- [ ] Remove `ImportBeats` mage target (if still present in `Magefile.go`)
- [ ] Remove any `import-beats`-exclusive entries from `tools.go` and run `go mod tidy` to prune unused dependencies from `go.mod` / `go.sum`
- To be validated that `github.com/elastic/package-registry` could be removed from tools.go along with other dependencies.
- [ ] Remove docs related to `import-beats` tool
- [ ] Check that no CI step or documentation references the script