Revert "src/core/screen.c: Set _NET_NUMBER_OF_DESKTOPS in meta_screen_new"

When dynamic workspaces are used, we pick up an existing NUMBER_OF_DESKTOP
hint in meta_screen_init_workspaces() to properly restore workspaces
on restart. Unconditionally setting that hint to 1 *before* reading it
breaks that, and we end up shifting all windows to the first workspace.

This reverts commit 8532b102.

https://bugzilla.gnome.org/show_bug.cgi?id=760651