Remove dataset package

[stephen-soltesz]

This change removes the dataset package from m-lab/go. There are no remaining references to this package within any searchable github.com/m-lab repo. https://github.com/search?q=org%3Am-lab+%22go%2Fdataset%22&type=code

Removing this package is part of:

• https://github.com/m-lab/go/issues/88

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1268

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 94.647%

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Totals
Change from base Build 1266:	0.04%
Covered Lines:	2228
Relevant Lines:	2354

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💛 - Coveralls