soc: stm32f3x: clean up after Cube LL clock control

Following activation of Cube LL based clock control driver,
this commits cleans up the useless structures for RCC definitions
and remove code relative to native F3 Clock control driver.

Change-Id: I6f3ee44adb09adc52927eb4b05f8a829665eb96d
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>