Revert "gl: cocoa: sprinkle some GST_EXPORT"

This reverts commit 94d798c3337013bd902c0fd0a66f7f6842243eeb.

cocoac headers don't need to be public as all the functionality can be
provided by the base class

gst-libs/gst/gl/cocoa/gstgldisplay_cocoa.h

@@ -28,7 +28,6 @@
 
 G_BEGIN_DECLS
 
-GST_EXPORT
 GType gst_gl_display_cocoa_get_type (void);
 
 #define GST_TYPE_GL_DISPLAY_COCOA             (gst_gl_display_cocoa_get_type())
@@ -56,7 +55,6 @@ struct _GstGLDisplayCocoaClass
   GstGLDisplayClass object_class;
 };
 
-GST_EXPORT
 GstGLDisplayCocoa *gst_gl_display_cocoa_new (void);
 
 G_END_DECLS