clc: remove code supporting pre llvm-10
we require llvm-10+ already anyway, see https://gitlab.freedesktop.org/mesa/mesa/-/blob/0ffb828513d0225e3657e21cefb496aba499033f/meson.build#L1727
we require llvm-10+ already anyway, see https://gitlab.freedesktop.org/mesa/mesa/-/blob/0ffb828513d0225e3657e21cefb496aba499033f/meson.build#L1727