wayland: don't ignore key modifiers if they were consumed

According to the xkbcommon docs, `xkb_state_mod_index_is_consumed` is
true when a modifier *may affect* key translation. A key modifier may
be consumed but not be active. See xkb documentation for this function
for further details. This breaks key modifiers in cases where
L_Shift+R_Shift for example is used to change keyboard layout with
`xkb_options grp:shifts_toggle`. Instead, replace it with a simple
check for a valid modifier.