[C++] Remove duplicate library in cpp/Brewfile

[asfimport]

Reporter: Yosuke Shiro / @shiro615
Assignee: Yosuke Shiro / @shiro615

PRs and other links:

• GitHub Pull Request #4816

_{Note: This issue was originally created as ARROW-5866. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 4816
#4816