kbuild: remove redundant LDFLAGS clearing in arch/*/Makefile

Since commit ce99d0bf ("kbuild: clear LDFLAGS in the top Makefile"),
the top-level Makefile caters to this.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>