Replace golang.org/x/net/context with context package
go fix these changes were made, I only applied some regexp
to get the import grouping correct.
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.