[Packaging] Remove unused section from packing spec

[ycombinator]

What does this PR do?

This PR removes the MacOS section from the packaging spec file because that section is never actually used in packaging.

Why is it important?

To avoid confusion.

How to test this PR locally

1. Package Elastic Agent with this PR.

   SNAPSHOT=true PLATFORMS=darwin/arm64 PACKAGES=targz mage package

2. Unpack and install agent.

   cd /tmp
   tar xzf $OLDPWD/build/distributions/elastic-agent-8.13.0-SNAPSHOT-darwin-aarch64.tar.gz
   cd elastic-agent-8.13.0-SNAPSHOT-darwin-aarch64
   sudo ./elastic-agent install -n

3. Check that the Agent reports a healthy status.

   sudo elastic-agent status

4. Check that uninstallation works.

   sudo elastic-agent uninstall -f

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[pchila]

This looks like a leftover from whenever agent/beats packages for MacOS were created.
Since it's not used in current packaging we can safely get rid of it

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube