lib/string.c: drop pointless __HAVE_ARCH_STRDUP

There has never been an arch-specific optimized implementation of
str[n]dup, nor is there likely to ever be one, because unlike their
cousins strlen(), strcpy() and similar that simply read/write the
src/dst, the dup functions by definition involve memory allocation. So
drop this irrelevant cpp guard.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>

include/linux/string.h

@@ -101,10 +101,9 @@ size_t strcspn(const char *s, const char *reject);
 # define strndup		sandbox_strndup
 #endif
 
-#ifndef __HAVE_ARCH_STRDUP
 extern char * strdup(const char *);
 extern char * strndup(const char *, size_t);
-#endif
+
 #ifndef __HAVE_ARCH_STRSWAB
 extern char * strswab(const char *);
 #endif