ARM: spear: Remove references to PLAT_SPEAR_SINGLE

The Kconfig symbol PLAT_SPEAR_SINGLE briefly appeared during the v3.10
development cycle. It was removed in a merge commit before v3.10. A few
references to it were left in the tree, probably because they didn't
generate merge conflicts. Whatever it was, they're useless now and can
safely be removed.

Reported-by: Martin Walch <walch.martin@web.de>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Reviewed-by: Rajeev Kumar <rajeev-dlh.kumar@st.com>
Cc: Rajeev Kumar <rajeev_kumar@mentor.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>