ArmPkg/ArmMmuLib: drop unused TT_ATTR_INDX_INVALID CPP macro

ArmPkg/ArmMmuLib: drop unused TT_ATTR_INDX_INVALID CPP macro

TT_ATTR_INDX_INVALID is #define'd but never used so drop it. Note
that this leaves a CPP macro of the same name in CpuDxe, but there,
it is actually being used, and although the name suggests that this
value is somehow defined by the architecture, this is really not the
case and it only has meaning within the scope of CpuDxe's implementation.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>