powerpc/cell: Drop CONFIG_TUNE_CELL in favour of CONFIG_CELL_CPU

The TUNE_CELL option allows you to build a kernel that runs on multiple
CPUs but is tuned (ie. optimised) to run on Cell CPUs. Now days no one
is building a distro in that fashion, and any users who are building
custom kernels for their Cell machines are better off building with
CONFIG_CELL_CPU, which builds a kernel that only runs on Cell and
therefore can be optimised even more aggresively.

Dropping the option also avoids confusing other users, who are presented
with an option to tune for Cell when they are not building for a Cell
CPU at all.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>