Removes index() from bionic.

This function has been removed from POSIX.

Unfortunately, we can't leave #define index(a, b) strchr((a), (b)) in its place
because defining a preprocessor macro for index() breaks a whole lot of code.

Bug: 13935372
Change-Id: Ifda348acde06da61c12e7ee2f8fe6950a3174dd1

libc/include/strings.h

@@ -47,7 +47,6 @@ __BEGIN_DECLS
 #define bzero(b, len) (void)(memset((b), '\0', (len)))
 
 int	 ffs(int);
-char	*index(const char *, int);
 int	 strcasecmp(const char *, const char *);
 int	 strncasecmp(const char *, const char *, size_t);