phy: Remove unused phy_optional_get()

There were never any upstream users of this function since its
introduction almost 10 years ago.

Besides, the dummy for phy_optional_get() should have returned NULL
instead of an error code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/df61992b1d66bccf4e6e1eafae94a7f7d7629f34.1674584626.git.geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>