string: remove unused strswab() function

The last use of this function with rather peculiar semantics[*] vanished
in 2021 with 0a527fda78 ("Fix IDE commands issued, fix endian issues,
fix non MMIO"). It has no tests, and should a need for something
similar ever appear, it is better done with some proper
utf16le/utf16be/utf16 abstractions rather than cluttering code with
'#ifdef __LITTLE_ENDIAN'.

[*] The byte-swapping itself is weird enough. But why is an input string
of odd length ok, while the empty string is not allowed?

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

include/linux/string.h

@@ -107,10 +107,6 @@ extern char * strndup(const char *, size_t);
 extern const char *strdup_const(const char *s);
 extern void kfree_const(const void *x);
 
-#ifndef __HAVE_ARCH_STRSWAB
-extern char * strswab(const char *);
-#endif
-
 #ifndef __HAVE_ARCH_MEMSET
 extern void * memset(void *,int,__kernel_size_t);
 #endif