clk: Remove clk_{register,unregister}_multiplier()

These APIs aren't used, so remove them. This can be reverted if
we get a user at some point.

Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Suggested-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>