Fix deadcode linter warnings

Fix the following warnings:

call.go:8:5: `errSignature` is unused (deadcode)
var errSignature = errors.New("dbus: mismatched signature")
    ^
dbus.go:13:2: `uint8Type` is unused (deadcode)
	uint8Type       = reflect.TypeOf(uint8(0))
	^
dbus.go:16:2: `intType` is unused (deadcode)
	intType         = reflect.TypeOf(int(0))
	^
dbus.go:17:2: `uintType` is unused (deadcode)
	uintType        = reflect.TypeOf(uint(0))
	^

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

call.go

@@ -2,11 +2,8 @@ package dbus
 
 import (
 	"context"
-	"errors"
 )
 
-var errSignature = errors.New("dbus: mismatched signature")
-
 // Call represents a pending or completed method call.
 type Call struct {
 	Destination string

dbus.go

@@ -10,11 +10,8 @@ import (
 var (
 	byteType        = reflect.TypeOf(byte(0))
 	boolType        = reflect.TypeOf(false)
-	uint8Type       = reflect.TypeOf(uint8(0))
 	int16Type       = reflect.TypeOf(int16(0))
 	uint16Type      = reflect.TypeOf(uint16(0))
-	intType         = reflect.TypeOf(int(0))
-	uintType        = reflect.TypeOf(uint(0))
 	int32Type       = reflect.TypeOf(int32(0))
 	uint32Type      = reflect.TypeOf(uint32(0))
 	int64Type       = reflect.TypeOf(int64(0))