xtensa: remove redefinition of XCHAL_MMU_ASID_BITS

This constant is defined in all core headers.  Remove the redundant
definition which might error out if other includes lead to inclusion
of <variant/core.h>.

Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>