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>