gdrive: add gc command support

[maxhora]

Implements RemoteGDrive.remove to support gc functionality.

Related to #2865

[codecov]

Codecov Report

Merging #3288 into master will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3288      +/-   ##
==========================================
- Coverage   92.92%   92.56%   -0.36%     
==========================================
  Files         139      139              
  Lines        8493     8500       +7     
==========================================
- Hits         7892     7868      -24     
- Misses        601      632      +31     

Impacted Files	Coverage Δ
dvc/system.py	78.07% <0.00%> (-9.65%)	⬇️
dvc/remote/ssh/connection.py	82.50% <0.00%> (-4.50%)	⬇️
dvc/remote/gdrive.py	85.57% <0.00%> (-3.98%)	⬇️
dvc/analytics.py	94.52% <0.00%> (-1.37%)	⬇️
dvc/updater.py	54.65% <0.00%> (-1.17%)	⬇️

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[efiop]

@Maxris Please check the tests.

[maxhora]

@efiop it seems the exceptions are related to recent changes in commit 19acded#diff-24be7b52326b428c2569724796020192

error in Travis logs - https://travis-ci.com/iterative/dvc/jobs/285154054#L17901

not sure why it fails randomly, but just restart of failed jobs helped to pass everything successfully

[efiop]

@Maxris Thanks for investigating! I'll take a look.