main: Stop setting GOMAXPROCS to NumCPU explicitly (#1096)

GOMAXPROCS will be set automatically after Go 1.5. The minimum version
supported at the moment is 1.9, so setting GOMAXPROCS to NumCPU is not
necessary anymore.

References
https://docs.google.com/document/d/1At2Ls5_fhJQ59kDK2DFVhFu3g5mATSXqqV5QrxinasI/edit
https://tip.golang.org/doc/go1.5#introduction

lnd.go

@@ -21,7 +21,6 @@ import (
 	_ "net/http/pprof"
 	"os"
 	"path/filepath"
-	"runtime"
 	"runtime/pprof"
 	"strings"
 	"sync"
@@ -614,10 +613,6 @@ func lndMain() error {
 }
 
 func main() {
-	// Use all processor cores.
-	// TODO(roasbeef): remove this if required version # is > 1.6?
-	runtime.GOMAXPROCS(runtime.NumCPU())
-
 	// Call the "real" main in a nested manner so the defers will properly
 	// be executed in the case of a graceful shutdown.
 	if err := lndMain(); err != nil {