stm32l4: rcc: drop bad function prototype

This was introduced by a bad merge/rebase leaving a dangling header
definition.

Fixes: b8424263 stm32:l4: rcc: Add RTC clock functions

include/libopencm3/stm32/l4/rcc.h

@@ -964,7 +964,6 @@ void rcc_set_msi_range(uint32_t msi_range);
 void rcc_set_msi_range_standby(uint32_t msi_range);
 void rcc_pll_output_enable(uint32_t pllout);
 void rcc_set_clock48_source(uint32_t clksel);
-void rcc_clock_setup_pll(const struct rcc_clock_scale *clock);
 void rcc_enable_rtc_clock(void);
 void rcc_disable_rtc_clock(void);
 void rcc_set_rtc_clock_source(enum rcc_osc clk);