[C++] Necessity of the build-support scripts in the cpp directory (ShellCheck)

[hiroyuki-sato]

Describe the enhancement requested

This is the sub issue #44748.

It looks like the following three files are not referenced anywhere else in this project.
Would it be safe to remove them, or should I fix the ShellCheck errors instead?
Depending on the answer, I'll adjust the contents of the PR accordingly.

• cpp/build-support/trim-boost.sh
• cpp/build-support/update-flatbuffers.sh
• cpp/build-support/vendor-flatbuffers.sh

Component(s)

C++

[kou]

We can remove trim-boost.sh.

Let's keep *-flatbuffer.sh for now. We may update Flatbuffers related files later #33031. Or we may support system Flatbuffers #45636.