GH-1305: [CI] Drop x86_64 macOS from JNI builds

[jbonofre]

Rationale for this change

Homebrew treats x86_64 macOS as a tier 3 configuration and rarely builds bottles for it. brew bundle doesn't build from source, so the JNI macos-15-intel x86_64 job in the RC workflow aborts in Install dependencies with no bottle available! before anything is compiled.

We've been patching this formula by formula:

• MINOR: Don't install Homebrew's aws-sdk-cpp and gRPC for JNI macOS build #1269 skipped aws-sdk-cpp and gRPC.
• MINOR: Don't install Homebrew's Node.js for JNI macOS build #1270 skipped Node.js.

Any new dependency added to apache/arrow's cpp/Brewfile can break the job again the same way, and nightly RC runs go red on main when it does. We can't keep this job green on a configuration Homebrew doesn't support.

#1269 noted that dropping the macos-15-intel entry needed its own discussion because it's user-facing. #1305 is that discussion.

What changes are included in this PR?

rc.yml:

• Removed the macos-15-intel / x86_64 entry from the jni-macos matrix.
• binaries no longer extracts jni-macos-x86_64.tar.gz nor asserts the four jni/*/x86_64/*.dylib files.
• Dropped the /usr/local python@XXX --overwrite conflict loop. Its comment already said it could go once we dropped macos-15-intel, since macos-14 and later on arm64 use /opt/homebrew. brew update is kept.
• Unwrapped the pkg-config uninstall from its if [ "$(uname -m)" = "arm64" ] guard, which is now always true.
• Trimmed the tier-3 justification from the HOMEBREW_BUNDLE_BREW_SKIP comment. The skip list itself is unchanged: aws-sdk-cpp and gRPC are still skipped so the bundled static ones are used, and Node.js is still skipped because the JNI build doesn't need it.

test.yml:

• Removed the AMD64 / macos-15-intel entry from the macos test matrix, leaving AArch64 on macos-latest.

ci/scripts/jni_macos_build.sh is untouched, so building the JNI libraries locally on an Intel Mac still works.

Are these changes tested?

Yes, by CI. The remaining JNI macos-14 aarch_64 job exercises the changed Install dependencies step, and binaries exercises the changed artifact extraction and verification.

Are there any user-facing changes?

Yes. Release JARs no longer bundle x86_64 macOS .dylib files for arrow_cdata_jni, arrow_dataset_jni, arrow_orc_jni and gandiva_jni. Intel Mac users of those modules will need to build the JNI libraries themselves. aarch64 macOS, x86_64 and aarch64 Linux, and x86_64 Windows are unaffected.

Closes #1305.

[kou]

+1