all: fix some issues spotted by static analysis

First, vet was upset by two incorrect fmt verbs. One was an extra %x,
when there was just one argument, so remove it. Another was a %p with a
non-pointer. It's a struct, so for now simply use %#v.

Second, staticcheck found some unused or unnecessary bits of code;
remove the obvious ones.

Finally, staticcheck also complained about some error strings which were
capitalized or had periods. Adjust those, which also makes all error
messages more consistent.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

ui/app/window.go

@@ -361,4 +361,4 @@ func WithHeight(h ui.Value) WindowOption {
 	}
 }
 
-func (_ driverEvent) ImplementsEvent() {}
+func (driverEvent) ImplementsEvent() {}