kbuild: remove a special handling for *.agh in Makefile.headersinst

scripts/Makefile.headersinst takes care of *.agh just for

  arch/cris/include/uapi/arch-v10/arch/sv_addr.agh

because renaming exported headers is difficult (or impossible).

This code is no longer necessary thanks to commit c690eddc ("CRIS:
Drop support for the CRIS port").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>