[libc] Remove unused le32_to_cpup(x) and cpu_to_le16p() macros

If we ever need these macros, they should be defined in
include/{big,little}_bswap.h, and the whole family should be defined.

src/arch/i386/include/bits/endian.h

 
 #define __BYTE_ORDER __LITTLE_ENDIAN
 
-#define le32_to_cpup(x) (*(uint32_t *)(x))
-#define cpu_to_le16p(x) (*(uint16_t*)(x))
-
 #endif /* ETHERBOOT_BITS_ENDIAN_H */