s390/vx: remove __uint128_t type from __vector128 struct again

s390/vx: remove __uint128_t type from __vector128 struct again

The __uint128_t member was only added for future convenience to the
__vector128 struct. However this is a uapi header file, 31/32 bit (aka
compat layer) is still supported, but doesn't know anything about this
type:

/usr/include/asm/types.h:27:17: error: unknown type name __uint128_t
   27 |                 __uint128_t v;

Therefore remove it again.

Fixes: b0b7b43fcc46 ("s390/vx: add 64 and 128 bit members to __vector128 struct")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>