Revert "[MMFMWK-7873] glimagesink: need add glFinish after eglswapbuffer"

This is the wrong way to fix tearing as it will stall the rendering
pipeline. https://bugzilla.gnome.org/show_bug.cgi?id=791937

No tearing visible with this reverted, but performance goes up from
23 to 29 fps in one particular test.

This reverts commit d20d23dc4cb100479d1edb627b4cf14d45998923.
diff --git a/gst-libs/gst/gl/egl/gstglcontext_egl.c b/gst-libs/gst/gl/egl/gstglcontext_egl.c
index 6d9ca2b..a8cf2f8 100644
--- a/gst-libs/gst/gl/egl/gstglcontext_egl.c
+++ b/gst-libs/gst/gl/egl/gstglcontext_egl.c
@@ -671,13 +671,10 @@
 gst_gl_context_egl_swap_buffers (GstGLContext * context)
 {
   GstGLContextEGL *egl;
-  const GstGLFuncs *gl;
 
   egl = GST_GL_CONTEXT_EGL (context);
-  gl = context->gl_vtable;
 
   eglSwapBuffers (egl->egl_display, egl->egl_surface);
-  gl->Finish();
 }
 
 static GstGLAPI