Remove Portenta-specific system clock override as this can be done through targets.json now

Author: multiplemonomials

targets/TARGET_STM/TARGET_STM32H7/TARGET_STM32H747xI/TARGET_PORTENTA_H7/CMakeLists.txt

@@ -8,7 +8,6 @@ add_library(mbed-portenta-h7-m4 INTERFACE)
 target_sources(mbed-portenta-h7-m4
     INTERFACE
         PeripheralPins.c
-        system_clock_override.c
         portenta_power.cpp
 )
 
@@ -25,7 +24,6 @@ add_library(mbed-portenta-h7-m7 INTERFACE)
 target_sources(mbed-portenta-h7-m7
     INTERFACE
         PeripheralPins.c
-        system_clock_override.c
         portenta_power.cpp
 )