Revert flash_get_value for M2351 done in PR#8589

`flash_get_erase_value` is added in hal/TARGET_FLASH_CMSIS_ALGO/flash_common_algo.c
duplicate copy of same function in targets/TARGET_NUVOTON/TARGET_M2351/flash_api.c
resulted in build issues with secure build

Author: Deepika

targets/TARGET_NUVOTON/TARGET_M2351/flash_api.c

@@ -134,12 +134,4 @@ void flash_set_target_config(flash_t *obj)
 }
 
 #endif  // #if defined (__ARM_FEATURE_CMSE) && (__ARM_FEATURE_CMSE == 3U)
-
-MBED_NONSECURE_ENTRY uint8_t flash_get_erase_value(const flash_t *obj)
-{
-    (void)obj;
-
-    return 0xFF;
-}
-
 #endif  // #if DEVICE_FLASH