Remove unused string.h include.

This is unused now that we use the silly memcpy, etc., wrappers to work
around the C NULL/0 language bug.

See https://android-review.googlesource.com/c/platform/external/boringssl/+/670794

Change-Id: I15c878cee6badb4551c8d5cfa1371a9bff4000fb
Reviewed-on: https://boringssl-review.googlesource.com/27666
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/crypto/mem.c b/crypto/mem.c
index 50c6fe6..5d45baa 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -59,7 +59,6 @@
 #include <assert.h>
 #include <stdarg.h>
 #include <stdio.h>
-#include <string.h>
 
 #if defined(OPENSSL_WINDOWS)
 OPENSSL_MSVC_PRAGMA(warning(push, 3))