efi_selftest: remove redundant function efi_st_memcmp()

Function memcmp() is available in efi_freestanding.c. So we do not remove a
further implementation.

Replace all usages of efi_st_memcmp() by memcmp().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

include/efi_selftest.h

@@ -92,17 +92,6 @@ u16 *efi_st_translate_char(u16 code);
  */
 u16 *efi_st_translate_code(u16 code);
 
-/*
- * Compare memory.
- * We cannot use lib/string.c due to different CFLAGS values.
- *
- * @buf1:	first buffer
- * @buf2:	second buffer
- * @length:	number of bytes to compare
- * @return:	0 if both buffers contain the same bytes
- */
-int efi_st_memcmp(const void *buf1, const void *buf2, size_t length);
-
 /*
  * Compare an u16 string to a char string.
  *