mbed_assert_failure: Don't take critical section

Assert failure took a critical section before calling `mbed_error`.

There's no need to take a critical section on assert failure -
mbed_error does not do this, and is designed to operate from normal
contexts.

Avoiding the critical section will improve the chances of console
initialisation due to assert failure working nicely.

platform/mbed_assert.c

@@ -23,7 +23,6 @@
 
 MBED_NORETURN void mbed_assert_internal(const char *expr, const char *file, int line)
 {
-    core_util_critical_section_enter();
     mbed_error(MBED_ERROR_ASSERTION_FAILED, expr, 0, file, line);
 }