gpu/gl: remove unused field

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/gl/backend.go

@@ -103,7 +103,6 @@ type uniformLocation struct {
 }
 
 type gpuInputLayout struct {
-	backend *Backend
 	inputs []backend.InputLocation
 	layout []backend.InputDesc
 }
@@ -416,7 +415,6 @@ func (b *Backend) NewInputLayout(vs backend.ShaderSources, layout []backend.Inpu
 		}
 	}
 	return &gpuInputLayout{
-		backend: b,
 		inputs: vs.Inputs,
 		layout: layout,
 	}, nil