[M252KG] Remove TRNG support

Reasons to remove TRNG support:
1.  M252 just has 32KiB SRAM and cannot afford mbedtls application.
2.  Implementing TRNG HAL with PRNG H/W has security concern.

Author: ccli8

targets/TARGET_NUVOTON/TARGET_M251/crypto/crypto-misc.cpp

@@ -126,10 +126,6 @@ struct sleep_s {
     int powerdown;
 };
 
-struct trng_s {
-    uint8_t dummy;
-};
-
 #ifdef __cplusplus
 }
 #endif