systimer tests: do not require lp ticker

SysTimer can run on us ticker now. Only deep sleep test
strictly requires lp ticker and it already has build conditions
in place.

TESTS/mbedmicro-rtos-mbed/systimer/main.cpp

@@ -17,10 +17,6 @@
 #error [NOT_SUPPORTED] Tickless mode not supported for this target.
 #endif
 
-#if !DEVICE_LPTICKER
-#error [NOT_SUPPORTED] Current SysTimer implementation requires lp ticker support.
-#endif
-
 #include "mbed.h"
 #include "greentea-client/test_env.h"
 #include "unity.h"