[Dev][Docs] dev/merge_arrow_pr.py doesn't require "arrow" remote #38300
Description
Describe the enhancement requested
https://github.com/apache/arrow/tree/main/dev#how-to-merge-a-pull-request says that we need "arrow" remote but it's no longer needed. Because we migrated to GitHub API from local Git merge by #13184.
Component(s)
Developer Tools, Documentation