um: remove ARCH_NO_PREEMPT_DYNAMIC

There's no such symbol and we currently don't have any of the
mechanisms to make boot-time selection cheap enough, so we can't
have HAVE_PREEMPT_DYNAMIC_CALL or HAVE_PREEMPT_DYNAMIC_KEY.

Remove the select statement.

Reported-by: Lukas Bulwahn <lbulwahn@redhat.com>
Fixes: cd01672d ("um: Enable preemption in UML")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>