removed sync.Mutex from properties type

it's not necessary and in fact is an error to make copies of a
sync.Mutex due to how we're using instances of the properties type

properties/properties.go

@@ -20,7 +20,6 @@ import (
 	"fmt"
 	"os"
 	"strings"
-	"sync"
 
 	"github.com/jetsetilly/gopher2600/logger"
 	"github.com/jetsetilly/gopher2600/resources"
@@ -42,7 +41,6 @@ func (e Entry) IsValid() bool {
 }
 
 type Properties struct {
-	crit      sync.Mutex
 	available bool
 	entries   map[string]Entry
 }
@@ -127,9 +125,6 @@ func Load() (Properties, error) {
 
 // Find the property entry for the ROM with the supplied md5 hash
 func (pro Properties) Lookup(md5Hash string) Entry {
-	pro.crit.TryLock()
-	defer pro.crit.Unlock()
-
 	if e, ok := pro.entries[md5Hash]; ok {
 		return e
 	}