FIX: LPUART clock source selection should be left to serial driver

The clock source selection of LPUART depends on System clocks but also on
the serial baudrate. There is a specific computation done in serial driver
targets/target_STM/serial_api.c

At first start-up the LPUART1 clock selected in SetSysClock was anyway
overridden by the serial driver, so this was of no effect. But in case
of deep sleep SetSysClock is called again, while the driver isn't, so
SetSyClock was corrupting the serial clock configuration.

So let's remove these few lines of code which are causing trouble.

Author: LMESTM

targets/TARGET_STM/TARGET_STM32L4/TARGET_STM32L4R5xI/TARGET_NUCLEO_L4R5ZI/system_clock.c

@@ -177,13 +177,6 @@ uint8_t SetSysClock_PLL_HSE(uint8_t bypass)
     RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE; // No PLL update
     HAL_RCC_OscConfig(&RCC_OscInitStruct);
 
-    /* Select HSI as clock source for LPUART1 */
-    RCC_PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
-    RCC_PeriphClkInit.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_HSI;
-    if (HAL_RCCEx_PeriphCLKConfig(&RCC_PeriphClkInit) != HAL_OK) {
-        return 0; // FAIL
-    }
-
     // Output clock on MCO1 pin(PA8) for debugging purpose
 #if DEBUG_MCO == 2
     if (bypass == 0) {
@@ -259,13 +252,6 @@ uint8_t SetSysClock_PLL_HSI(void)
     RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE; // No PLL update
     HAL_RCC_OscConfig(&RCC_OscInitStruct);
 
-    /* Select HSI as clock source for LPUART1 */
-    RCC_PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
-    RCC_PeriphClkInit.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_HSI;
-    if (HAL_RCCEx_PeriphCLKConfig(&RCC_PeriphClkInit) != HAL_OK) {
-        return 0; // FAIL
-    }
-
     // Output clock on MCO1 pin(PA8) for debugging purpose
 #if DEBUG_MCO == 3
     HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_HSI, RCC_MCODIV_1); // 16 MHz
@@ -331,13 +317,6 @@ uint8_t SetSysClock_PLL_MSI(void)
         return 0; // FAIL
     }
 
-    /* Select LSE as clock source for LPUART1 */
-    PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_LPUART1;
-    PeriphClkInitStruct.Lpuart1ClockSelection = RCC_LPUART1CLKSOURCE_LSE;
-    if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK) {
-        return 0; // FAIL
-    }
-
     // Output clock on MCO1 pin(PA8) for debugging purpose
 #if DEBUG_MCO == 4
     HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_MSI, RCC_MCODIV_2); // 2 MHz