Remove homebrew from docs to set up runner on MacOS

What does this MR do?

The documentation for setting up a MacOS GitLab Runner provides Homebrew as an installation method, which is inconsistent with the statement that the binary is the official method for installing GitLab Runner.

This removes reference to using homebrew in the following pages:

• https://docs.gitlab.com/runner/install/osx.html

Before: https://docs.gitlab.com/runner/install/osx.html#homebrew-installation-alternative

After: https://main-runner-4227.docs.gitlab-review.app/runner/install/osx.html

Related issues

gitlab#415392 (closed)

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