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>

lib/string.c

@@ -343,7 +343,6 @@ size_t strcspn(const char *s, const char *reject)
 }
 #endif
 
-#ifndef __HAVE_ARCH_STRDUP
 char * strdup(const char *s)
 {
 	char *new;
@@ -379,7 +378,6 @@ char * strndup(const char *s, size_t n)
 
 	return new;
 }
-#endif
 
 #ifndef __HAVE_ARCH_STRSPN
 /**