Enable all linters by default

This removes the directive from .golangci.yml to disable all linters,
and fixes the relevant bugs / issues that are exposed.

trace/opencensus/opencensus_test.go

@@ -15,13 +15,8 @@
 package opencensus
 
 import (
-	"testing"
-
 	"github.com/virtual-kubelet/virtual-kubelet/trace"
 )
 
-func TestTracerImplementsTracer(t *testing.T) {
-	// ensure that Adapter implements trace.Tracer
-	if tt := trace.Tracer(Adapter{}); tt == nil {
-	}
-}
+// ensure that Adapter implements trace.Tracer
+var _ trace.Tracer = (*Adapter)(nil)