[CI] Remove needless LLVM_DIR for macOS

[asfimport]

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #3674

_{Note: This issue was originally created as ARROW-4598. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 3674
#3674