estarlight: remove, since now available upstream (#258)

### Briefly, what does this PR introduce?
This removes the estarlight package, since it is being upstreamed at
spack/spack#32834.

### What kind of change does this PR introduce?
- [ ] Bug fix (issue #__)
- [X] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [X] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
Users of estarlight will see this disappear when updating if they do not
use `eic-spack` with a `spack:develop` that is at least as recent.

### Does this PR change default behavior?
No.