fix(definitions): remove RenderContext interface

not needed for typescript builds after all, see:
https://github.com/opensheetmusicdisplay/opensheetmusicdisplay/issues/448#issuecomment-436941608

external/vexflow/vexflow.d.ts

@@ -373,26 +373,6 @@ declare namespace Vex {
             public lineWidth: number;
         }
 
-        /** Apparently, this interface is needed for a typescript build including OSMD
-         * see https://github.com/opensheetmusicdisplay/opensheetmusicdisplay/issues/448#issuecomment-436407775
-         */
-        export interface RenderContext {
-            scale(x: number, y: number): RenderContext;
-            fillRect(x: number, y: number, width: number, height: number): RenderContext
-            fillText(text: string, x: number, y: number): RenderContext;
-            setFont(family: string, size: number, weight: string): RenderContext;
-            beginPath(): RenderContext;
-            moveTo(x, y): RenderContext;
-            lineTo(x, y): RenderContext;
-            bezierCurveTo(cp1_x: number, cp1_y: number, cp2_x: number, cp2_y: number, end_x: number, end_y: number): RenderContext;
-            closePath(): RenderContext;
-            stroke(): RenderContext;
-            fill(): RenderContext;
-            save(): RenderContext;
-            restore(): RenderContext;
-            lineWidth: number;
-        }
-
         export class CanvasContext extends RenderContext {
             public vexFlowCanvasContext: CanvasRenderingContext2D;
         }