sched: dynamic: Make PREEMPT_DYNAMIC depend on ARCH_HAS_PREEMPT_LAZY

On architectures which select ARCH_HAS_PREEMPT_LAZY, it has not been
possible to select the NONE and VOLUNTARY preemption models since
commit:

  7dadeaa6e851 ("sched: Further restrict the preemption modes")

... which was merged in v7.0.

All architectures which currently support PREEMPT_DYNAMIC select
ARCH_HAS_PREEMPT_LAZY:

  [mark@lakrids:~/src/linux]% git describe HEAD
  v7.2-rc1-1-g871a4586ea2e1
  [mark@lakrids:~/src/linux]% git grep 'select HAVE_PREEMPT_DYNAMIC_' -- arch
  arch/arm64/Kconfig:     select HAVE_PREEMPT_DYNAMIC_KEY
  arch/loongarch/Kconfig: select HAVE_PREEMPT_DYNAMIC_KEY
  arch/powerpc/Kconfig:   select HAVE_PREEMPT_DYNAMIC_KEY
  arch/riscv/Kconfig:     select HAVE_PREEMPT_DYNAMIC_KEY
  arch/s390/Kconfig:      select HAVE_PREEMPT_DYNAMIC_KEY
  arch/x86/Kconfig:       select HAVE_PREEMPT_DYNAMIC_CALL
  [mark@lakrids:~/src/linux]% git grep 'select ARCH_HAS_PREEMPT_LAZY' -- arch
  arch/arm64/Kconfig:     select ARCH_HAS_PREEMPT_LAZY
  arch/loongarch/Kconfig: select ARCH_HAS_PREEMPT_LAZY
  arch/powerpc/Kconfig:   select ARCH_HAS_PREEMPT_LAZY
  arch/riscv/Kconfig:     select ARCH_HAS_PREEMPT_LAZY
  arch/s390/Kconfig:      select ARCH_HAS_PREEMPT_LAZY
  arch/x86/Kconfig:       select ARCH_HAS_PREEMPT_LAZY

... and hence in practice PREEMPT_DYNAMIC no longer supports the NONE or
VOLUNTARY preemption models.

Make this official: have PREEMPT_DYNAMIC depend on
ARCH_HAS_PREEMPT_LAZY, and remove the trivially unreachable code.
Further simplifications will be made in subsequent patches.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Tested-by: Mete Durlu <meted@linux.ibm.com>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Tested-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://patch.msgid.link/20260803191731.3244294-2-mark.rutland@arm.com