Remove unused Gopkg.toml constraint

[samb1729]

Resolves this warning when running dep ensure:

Warning: the following project(s) have [[constraint]] stanzas in Gopkg.toml:

  ✗  google.golang.org/grpc

However, these projects are not direct dependencies of the current project:
they are not imported in any .go files, nor are they in the 'required' list in
Gopkg.toml. Dep only applies [[constraint]] rules to direct dependencies, so
these rules will have no effect.

Either import/require packages from these projects so that they become direct
dependencies, or convert each [[constraint]] to an [[override]] to enforce rules
on these projects, if they happen to be transitive dependencies.

[squaremo]

I am not sure why this [[constraint]] is here at all -- Helm is the only thing using grpc so far as I know, so many it was a problem with that at one point.