ARROW-4598: [CI] Remove needless LLVM_DIR for macOS

[kou]

We can detect LLVM installed by Homebrew automatically in CMake.

[codecov-io]

Codecov Report

Merging #3674 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3674      +/-   ##
==========================================
- Coverage   87.78%   87.77%   -0.01%     
==========================================
  Files         689      689              
  Lines       84023    84023              
  Branches     1081     1081              
==========================================
- Hits        73757    73755       -2     
- Misses      10151    10153       +2     
  Partials      115      115

Impacted Files	Coverage Δ
js/src/ipc/metadata/json.ts	92.39% <0%> (-4.35%)	⬇️
cpp/src/plasma/thirdparty/ae/ae.c	72.03% <0%> (+0.94%)	⬆️

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[kszucs]

LGTM