GH-46067: [CI][C++] Remove system Flatbuffers from macOS (#46105)

### Rationale for this change

Flatbuffers statically requires that the version used to generate the prototypes within the code base is the same version used to compile it. A system install of Flatbuffers is therefore highly unlikely to be usable

### What changes are included in this PR?

Removed the system install of Flatbuffers on macOS

### Are these changes tested?

In CI

### Are there any user-facing changes?

No

* GitHub Issue: #46067

Authored-by: Will Ayd <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: WillAyd

cpp/Brewfile

@@ -24,7 +24,6 @@ brew "c-ares"
 brew "curl"
 brew "ccache"
 brew "cmake"
-brew "flatbuffers"
 brew "git"
 brew "glog"
 brew "googletest"