Remove variable source interface

Signed-off-by: Mikalai Radchuk <[email protected]>

pkg/deppy/input/variable_source.go → pkg/deppy/input/simple_variable.go

@@ -1,16 +1,9 @@
 package input
 
 import (
-	"context"
-
 	"github.com/operator-framework/deppy/pkg/deppy"
 )
 
-// VariableSource generates solver constraints given an entity querier interface
-type VariableSource interface {
-	GetVariables(ctx context.Context) ([]deppy.Variable, error)
-}
-
 var _ deppy.Variable = &SimpleVariable{}
 
 type SimpleVariable struct {