display: Remove meta_display_get_ignored_modifier_mask

It's unused, in both here and in gnome-shell.
display: Ignore the modmask passed into meta_display_begin_grab_op

A careful analysis of mutter's codebase shows that nothing actually
passes anything but 0 to this. gnome-shell has one instance, but it's
most likely a mistake.

Remove the grab_mask field and the one place in keybindings.c that uses it.

The parameter to begin_grab_op is left in for API compatibility reasons.
keybindings: Don't bother saving num_lock / caps_lock in MetaDisplay

It's unused outside of this one function.
keybindings: Eliminate the use of the stored modmap

We can simply check the XKB keysym here to see if it's a modifier.
keybindings: Use xkb_keysym_get_name
Use libxkbcommon keysym names everywhere
display: Remove code to calculate the above-tab keycode

We always know it will be KEY_GRAVE + 8.