MINOR: Don't uninstall Flatbuffers in CI

There is no more need after
apache/arrow@32641ec

Author: lidavidm

.github/workflows/rc.yml

@@ -260,11 +260,7 @@ jobs:
           # used on test  We uninstall Homebrew's Protobuf to ensure using
           # bundled Protobuf.
           brew uninstall protobuf
-
-          # We need Flatbuffers 24, not the latest version
-          # Homebrew does not offer older versions, so remove the Homebrew
-          # package and rely on Arrow using a bundled version instead
-          brew uninstall flatbuffers
+          brew install boost
 
           brew bundle --file=Brewfile
       - name: Prepare ccache