Remove Clock_driver_support_shutdown_hardware()

The aim of this clock driver hook was to stop clock tick interrupts at
some late point in the exit() procedure.

The use of atexit() pulls in malloc() which pulls in errno. It is
incompatible with the intention of the
CONFIGURE_DISABLE_NEWLIB_REENTRANCY configuration option.

The exit() function must be called from thread context, so accompanied
clock tick interrupts should cause no harm.  On the contrary, someone
may assume a normal operating system operation, e.g. working timeouts.

Remove the Clock_driver_support_shutdown_hardware() clock driver hook.

Close #3436.

bsps/lm32/shared/clock/ckinit.c

@@ -65,13 +65,6 @@ static void Clock_driver_support_initialize_hardware(void)
   lm32_interrupt_unmask(CLOCK_IRQMASK);
 }
 
-#define Clock_driver_support_shutdown_hardware() \
-  do { \
-    /* Disable clock interrupts and stop */ \
-    lm32_interrupt_unmask(CLOCK_IRQMASK); \
-    clockwrite(LM32_CLOCK_CR, LM32_CLOCK_CR_STOP); \
-  } while (0)
-
 #define CLOCK_DRIVER_USE_DUMMY_TIMECOUNTER
 
 #include "../../../shared/dev/clock/clockimpl.h"