poly1305-x86_64-cryptogams.S is a generated file, so it should be
cleaned up by 'make clean'.

Assigning it to the variable 'targets' teaches Kbuild that it is a
generated file. However, this line is not evaluated when cleaning
because scripts/Makefile.clean does not include include/config/auto.conf.

Remove the ifneq-conditional, so this file is correctly cleaned up.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Ingo Molnar <mingo@kernel.org>