sh: remove no-op macro VMLINUX_SYMBOL()

VMLINUX_SYMBOL() is no-op unless CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX
is defined.  It has ever been selected only by BLACKFIN and METAG.
VMLINUX_SYMBOL() is unneeded for SuperH-specific code.

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