export.h: remove VMLINUX_SYMBOL() and VMLINUX_SYMBOL_STR()

With the special case handling for Blackfin and Metag was removed by
commit 94e58e0a

 ("export.h: remove code for prefixing symbols with
underscore"), VMLINUX_SYMBOL() is no-op.

Replace the remaining usages, then remove the definition of
VMLINUX_SYMBOL() and VMLINUX_SYMBOL_STR().

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