asm-generic/mmu.h: Remove unused vmlist field from mm_context_t

Nothing is using the vmlist field in mm_context_t anymore. It has been removed
from the non-generic versions over 3 years ago 8feae131 ("NOMMU: Make VMAs per
MM as for MMU-mode linux").

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>