string.h: remove unused variable

(cherry picked from commit 48be71d02b6cc4e6493d38cdd6b7779032c38901)

Bug: 17784968
Change-Id: Iac7732fb4f7fe42977cb9f62472bb636e17e5232
diff --git a/libc/include/string.h b/libc/include/string.h
index 8df68e3..6384d57 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -98,7 +98,6 @@
 void* memcpy(void* __restrict dest, const void* __restrict src, size_t copy_amount) {
     char *d = (char *) dest;
     const char *s = (const char *) src;
-    size_t s_len = __bos0(s);
     size_t d_len = __bos0(d);
 
     return __builtin___memcpy_chk(dest, src, copy_amount, d_len);