STM32L4 : remove warning

This is the MBED current implementation, no need to warn

Author: jeromecoutant

targets/TARGET_STM/TARGET_STM32L4/STM32Cube_FW/STM32L4xx_HAL_Driver/Legacy/stm32l4xx_hal_can_legacy.c

@@ -104,7 +104,8 @@
 #error 'The HAL CAN driver cannot be used with its legacy, Please ensure to enable only one HAL CAN module at once in stm32l4xx_hal_conf.h file'
 #endif /* HAL_CAN_MODULE_ENABLED */
 
-#warning 'Legacy HAL CAN driver is enabled! It can be used with known limitations, refer to the release notes. However it is recommended to use rather the new HAL CAN driver'
+// MBED current implementation
+// #warning 'Legacy HAL CAN driver is enabled! It can be used with known limitations, refer to the release notes. However it is recommended to use rather the new HAL CAN driver'
 
 #if defined(CAN1)