Remove unused MBEDTLS_ERR_SSL_UNKNOWN_CIPHER

The SSL error code MBEDTLS_ERR_SSL_UNKNOWN_CIPHER is unused.

Remove it for Mbed TLS 3.0 and leave a comment indicating the
gap in the error code space it creates.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 7936f6b..6b42b22 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -107,7 +107,7 @@
  * SSL       5   2 (Started from 0x5F00)
  * CIPHER    6   8 (Started from 0x6080)
  * SSL       6   24 (Started from top, plus 0x6000)
- * SSL       7   32
+ * SSL       7   31 (Started from 0x7080, gap at 0x7300)
  *
  * Module dependent error code (5 bits 0x.00.-0x.F8.)
  */
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 2350910..aaddd7f 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -67,7 +67,7 @@
 #define MBEDTLS_ERR_SSL_INVALID_MAC                       -0x7180  /**< Verification of the message MAC failed. */
 #define MBEDTLS_ERR_SSL_INVALID_RECORD                    -0x7200  /**< An invalid SSL record was received. */
 #define MBEDTLS_ERR_SSL_CONN_EOF                          -0x7280  /**< The connection indicated an EOF. */
-#define MBEDTLS_ERR_SSL_UNKNOWN_CIPHER                    -0x7300  /**< An unknown cipher was received. */
+/* NOTE: Error space gap */
 #define MBEDTLS_ERR_SSL_NO_CIPHER_CHOSEN                  -0x7380  /**< The server has no ciphersuites in common with the client. */
 #define MBEDTLS_ERR_SSL_NO_RNG                            -0x7400  /**< No RNG was provided to the SSL module. */
 #define MBEDTLS_ERR_SSL_NO_CLIENT_CERTIFICATE             -0x7480  /**< No client certification received from the client, but required by the authentication mode. */