x86/mm: Drop unused MAX_PHYSADDR_BITS

The macro is not used anywhere, and has an incorrect value (going by the
comment) on x86_64 since commit c898faf9 ("x86: 46 bit physical address
support on 64 bits")

To avoid confusion, just remove the definition.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200723231544.17274-2-nivedita@alum.mit.edu