MIPS: ralink: drop ralink_clk_init for mt7621

ralink_clk_init is only called in arch/mips/ralink/clk.c which isn't
compiled for mt7621. And it doesn't export a proper cpu clock.
Drop this unused function.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>