Remove unused texture file loading functions

Getting cubemaps and volume textures from file is very unlikely to ever
happen for our purposes.

libobs/graphics/graphics.c

@@ -693,22 +693,6 @@ texture_t gs_create_texture_from_file(const char *file, uint32_t flags)
 	return NULL;
 }
 
-texture_t gs_create_cubetexture_from_file(const char *file, uint32_t flags)
-{
-	/* TODO */
-	UNUSED_PARAMETER(file);
-	UNUSED_PARAMETER(flags);
-	return NULL;
-}
-
-texture_t gs_create_volumetexture_from_file(const char *file, uint32_t flags)
-{
-	/* TODO */
-	UNUSED_PARAMETER(file);
-	UNUSED_PARAMETER(flags);
-	return NULL;
-}
-
 static inline void assign_sprite_rect(float *start, float *end, float size,
 		bool flip)
 {

libobs/graphics/graphics.h

@@ -481,10 +481,6 @@ EXPORT shader_t gs_create_pixelshader_from_file(const char *file,
 
 EXPORT texture_t gs_create_texture_from_file(const char *file,
 		uint32_t flags);
-EXPORT texture_t gs_create_cubetexture_from_file(const char *flie,
-		uint32_t flags);
-EXPORT texture_t gs_create_volumetexture_from_file(const char *flie,
-		uint32_t flags);
 
 #define GS_FLIP_U (1<<0)
 #define GS_FLIP_V (1<<1)