README: remove description about CONFIG_USE_ARCH_MEMCPY/SET

These options are now described in the Kconfig help.  We do not want
to maintain duplicated documentation.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>