Remove unnecessary endianness difference

The current code results in the same output regardless of endianness.
Verified using a QEMU VM to emulate MIPS.

hash.c

@@ -10,14 +10,10 @@
 
 unsigned char* addr_to_byte(unsigned char *bytes, const uint32_t addr[8])
 {
-#if IS_LITTLE_ENDIAN==1
     int i;
     for (i = 0; i < 8; i++) {
         to_byte(bytes + i*4, addr[i], 4);
     }
-#else
-    memcpy(bytes, addr, 32);
-#endif
     return bytes;
 }
 
@@ -90,7 +86,6 @@ int hash_h(const xmss_params *params,
     set_key_and_mask(addr, 0);
     addr_to_byte(byte_addr, addr);
     prf(params, key, byte_addr, pub_seed, params->n);
-    // Use MSB order
     set_key_and_mask(addr, 1);
     addr_to_byte(byte_addr, addr);
     prf(params, bitmask, byte_addr, pub_seed, params->n);

hash.h

@@ -4,8 +4,6 @@
 #include <stdint.h>
 #include "params.h"
 
-#define IS_LITTLE_ENDIAN 1
-
 unsigned char* addr_to_byte(unsigned char *bytes, const uint32_t addr[8]);
 
 int prf(const xmss_params *params,