clk: samsung: Remove dead code

samsung_clk_register_pll2550x() function is not used anymore,
so remove its declaration.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>