Replace golang.org/x/net/context with context package

Leveraging go fix these changes were made, I only applied some regexp to get the import grouping correct.

The context package was added to the Go stdlib in Go 1.7. In the early days of Gitaly we still supported Go 1.6 so we had to use golang.org/x/net/context for compatibility reasons. Because the minimum supported Go version in Gitaly is now Go 1.10 we don't need to use golang.org/x/net/context anymore in our own code.