metag: remove SET_PERSONALITY()

Commit e72837e3 ("default
SET_PERSONALITY() in linux/elf.h").

The above commit moved the common definition of SET_PERSONALITY() in a
bunch of the arch headers to linux/elf.h. Metag shares that common
definition so remove it from arch/metag/include/asm/elf.h too.

Signed-off-by: James Hogan <james.hogan@imgtec.com>