soc: stm32l4x: 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 L4 Clock control driver.

Change-Id: Ic0408d5f5ec66651d46d13dfaf447da2666367cd
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>