devel/go-tools: update to 0.6.0

This update removes the cover utility, which has been superseeded by
"go tool cover" since Go 1.5.

Changelog: https://github.com/golang/tools/compare/v0.5.0...v0.6.0

Approved by: flo (mentor)
Differential Revision: https://reviews.freebsd.org/D38625