alpha: Remove redundant local asm header redirections

Remove a number of asm headers locally redirected to the respective
generic or generated versions.

For asm-offsets.h all that is needed is a Kbuild entry for the generic
version, and for div64.h, irq_regs.h and kdebug.h nothing is needed as
in their absence they will be redirected automatically according to
include/asm-generic/Kbuild.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Matt Turner <mattst88@gmail.com>

arch/alpha/include/asm/Kbuild

 # SPDX-License-Identifier: GPL-2.0
 
 generated-y += syscall_table.h
+generic-y += asm-offsets.h
 generic-y += export.h
 generic-y += kvm_para.h
 generic-y += mcs_spinlock.h

arch/alpha/include/asm/asm-offsets.h

-#include <generated/asm-offsets.h>

arch/alpha/include/asm/div64.h

-#include <asm-generic/div64.h>

arch/alpha/include/asm/irq_regs.h

-#include <asm-generic/irq_regs.h>

arch/alpha/include/asm/kdebug.h

-#include <asm-generic/kdebug.h>