Kill supportsGlClampToEdge, which hasn't actually been used for a while

Originally committed to SVN as r4248.

aegisub/src/video_out_gl.cpp

@@ -91,7 +91,6 @@ static bool TestTexture(int width, int height, GLint format) {
 VideoOutGL::VideoOutGL()
 :	maxTextureSize(0),
 	supportsRectangularTextures(false),
-	supportsGlClampToEdge(false),
 	internalFormat(0),
 	frameWidth(0),
 	frameHeight(0),

aegisub/src/video_out_gl.h

@@ -52,8 +52,6 @@ private:
 	int maxTextureSize;
 	/// Whether rectangular textures are supported by the user's graphics card
 	bool supportsRectangularTextures;
-	/// Whether GL_CLAMP_TO_EDGE is supported by the user's drivers
-	bool supportsGlClampToEdge;
 	/// The internalformat to use
 	int internalFormat;