lib/string.c: drop pointless __HAVE_ARCH_STRDUP

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>