remove Curl_strcasestr() since there is no code at all using this function!

lib/strequal.c

@@ -76,29 +76,6 @@ int curl_strnequal(const char *first, const char *second, size_t max)
 #endif
 }
 
-/*
- * Curl_strcasestr() finds the first occurrence of the substring needle in the
- * string haystack.  The terminating `\0' characters are not compared. The
- * matching is done CASE INSENSITIVE, which thus is the difference between
- * this and strstr().
- */
-char *Curl_strcasestr(const char *haystack, const char *needle)
-{
-#if defined(HAVE_STRCASESTR)
-  return strcasestr(haystack, needle);
-#else
-  size_t nlen = strlen(needle);
-  size_t hlen = strlen(haystack);
-
-  while(hlen-- >= nlen) {
-    if(curl_strnequal(haystack, needle, nlen))
-      return (char *)haystack;
-    haystack++;
-  }
-  return NULL;
-#endif
-}
-
 #ifndef HAVE_STRLCAT
 /*
  * The strlcat() function appends the NUL-terminated string src to the end