stm32: Remove SystemInit funcs, use stm32lib versions instead.

stm32lib now provides system_stm32XXxx.c source files for all MCU variants,
which includes SystemInit and prescaler tables.  Since these are quite
standard and don't need to be changed, switch to use them instead of custom
variants, making the start-up code cleaner.

The SystemInit code in stm32lib was checked and is equivalent to what is
removed from the stm32 port in this commit.

ports/stm32/main.c

@@ -368,6 +368,14 @@ STATIC uint update_reset_mode(uint reset_mode) {
 #endif
 
 void stm32_main(uint32_t reset_mode) {
+    #if !defined(STM32F0) && defined(MICROPY_HW_VTOR)
+    // Change IRQ vector table if configured differently
+    SCB->VTOR = MICROPY_HW_VTOR;
+    #endif
+
+    // Enable 8-byte stack alignment for IRQ handlers, in accord with EABI
+    SCB->CCR |= SCB_CCR_STKALIGN_Msk;
+
     // Check if bootloader should be entered instead of main application
     powerctrl_check_enter_bootloader();