Remove ShowDiffs from config struct

Following the improvements in #480, #485, #486, a tiny bit of cleanup for a lone, forgotten line.

Plus the typo had been bugging me since I first saw it.

config.go

@@ -65,7 +65,6 @@ type Configuration struct {
 	GitClone      bool   `json:"gitclone"`
 	Provides      bool   `json:"provides"`
 	PGPFetch      bool   `json:"pgpfetch"`
-	ShowDiffs     bool   `json:"showdifs"`
 	UpgradeMenu   bool   `json:"upgrademenu"`
 	CleanMenu     bool   `json:"cleanmenu"`
 	DiffMenu      bool   `json:"diffmenu"`