Remove min/maj version from SSL context if only one version enabled

If the minor/major version is enforced at compile-time, the `major_ver`
and `minor_ver` fields in `mbedtls_ssl_context` are redundant and can
be removed.
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 00b941d..43443bf 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -962,12 +962,22 @@
 
 static inline int mbedtls_ssl_get_minor_ver( mbedtls_ssl_context const *ssl )
 {
+#if !defined(MBEDTLS_SSL_CONF_FIXED_MINOR_VER)
     return( ssl->minor_ver );
+#else /* !MBEDTLS_SSL_CONF_FIXED_MINOR_VER */
+    ((void) ssl);
+    return( MBEDTLS_SSL_CONF_FIXED_MINOR_VER );
+#endif /* MBEDTLS_SSL_CONF_FIXED_MINOR_VER */
 }
 
 static inline int mbedtls_ssl_get_major_ver( mbedtls_ssl_context const *ssl )
 {
+#if !defined(MBEDTLS_SSL_CONF_FIXED_MAJOR_VER)
     return( ssl->major_ver );
+#else /* !MBEDTLS_SSL_CONF_FIXED_MAJOR_VER */
+    ((void) ssl);
+    return( MBEDTLS_SSL_CONF_FIXED_MAJOR_VER );
+#endif /* MBEDTLS_SSL_CONF_FIXED_MAJOR_VER */
 }
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)