ARROW-2815: [CI] Skip Java tests and style checks on C++ job [skip appveyor]

[pitrou]

This omits all warning and debug logs from previous Maven output.

[pitrou]

@xhochy

[wesm]

+1, thanks @pitrou! It's a little bit disappointing; I was hoping the active Java developers would do some work to improve the messy build, but this will do for now. Merge on green

[codecov-io]

Codecov Report

Merging #2378 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2378      +/-   ##
==========================================
- Coverage   84.51%   84.49%   -0.03%     
==========================================
  Files         293      293              
  Lines       45329    45329              
==========================================
- Hits        38312    38301      -11     
- Misses       6990     6997       +7     
- Partials       27       31       +4

Impacted Files	Coverage Δ
go/arrow/math/float64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/uint64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/memory/memory_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/int64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/memory/memory_amd64.go	28.57% <0%> (-14.29%)	⬇️
go/arrow/math/math_amd64.go	31.57% <0%> (-5.27%)	⬇️
cpp/src/arrow/util/thread-pool-test.cc	98.91% <0%> (-0.55%)	⬇️
go/arrow/math/float64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/int64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/uint64_amd64.go	33.33% <0%> (ø)	⬆️
... and 4 more

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[BryanCutler]

@pitrou you could use --quiet that will output nothing unless there is an error. Want me to add that? Also, if you still want to run tests here, using #2465 will change the log level to get rid of the debug messages.