c6x: remove unused parameter in Kconfig

This patch proposes to remove the TMS320C6X_CACHES_ON kernel configuration
parameter defined in arch/c6x/Kconfig, but used nowhere
in the makefiles and source code.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Acked by: Mark Salter <msalter@redhat.com>