remove brew c++ flag

Summary:

the c++ is not needed since the listed brew formulas no longer have this
option. It also raises a warning that may generate confusion.

Backport of core PR12027
https://github.com/bitcoin/bitcoin/pull/12027/files

Depends on D3905.

Expected to fix https://github.com/Bitcoin-ABC/bitcoin-abc/issues/338.

Test Plan: Read it, test it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3908