[ci] cut some macOS jobs

[jameslamb]

Moving to #7187 moved the project from a GItHub Enterprise account to a Team account, so we now get only 5 concurrent macOS CI jobs instead of 50 😬

https://docs.github.com/en/actions/reference/limits#job-concurrency-limits-for-github-hosted-runners

We currently run 17 macOS jobs on every commit, so we're really feeling that lower concurrency... I've seen it not be uncommon for CI to take over an hour to finish if even 2 PRs are active at the same time.

This proposes dropping 6 of those jobs:

• 3 python-package jobs
• 2 Python + MPI jobs
• 1 R job

For all of these, I targeted macos-15-intel... Apple isn't making new hardware with x86_64 chips (as far as I know), so I think it's more important to preserve CI coverage of macOS + arm64.

Those MPI jobs were especially unnecessary... these days, I think it's very unlikely that there are many folks running LightGBM distributed training using MPI on multiple x86_64 macOS boxes 😅