Remove unused error code MBEDTLS_ERR_SSL_CERTIFICATE_REQUIRED

After the previous commit, the code is no longer used and can be
removed for Mbed TLS 3.0.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 776013f..73d155b 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -108,8 +108,8 @@
  * CIPHER    6   8 (Started from 0x6080)
  * SSL       6   23 (Started from top, plus 0x6000, gaps at
  *                   0x6600)
- * SSL       7   29 (Started from 0x7080, gaps at
- *                   0x7300, 0x7500, 0x7800)
+ * SSL       7   28 (Started from 0x7080, gaps at
+ *                   0x7300, 0x7500, 0x7580, 0x7800)
  *
  * Module dependent error code (5 bits 0x.00.-0x.F8.)
  */