s390/percpu: remove this_cpu_cmpxchg_double_4

git commit 26f15caa ("s390/cmpxchg: simplify cmpxchg_double")
removed support for cmpxchg_double for two consecutive four byte
values, for which it would generate a cds instruction.

However I forgot to remove the corresponding define in our percpu
header file, which means that this_cpu_cmpxchg_double would now
incorrectly generate a cdsg instruction if being used on a double four
byte location. Therefore remove the percpu define as well.

There is currently no user and therefore no bug fixed with
this. Obviously any such user could and should simply use cmpxchg.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>