Fix long-standing bug in error code description

MBEDTLS_ERR_ASN1_INVALID_DATA is documented as "not used", but it has
been used since the PolarSSL days.
diff --git a/library/error.c b/library/error.c
index 7d7155b..649b3ba 100644
--- a/library/error.c
+++ b/library/error.c
@@ -467,7 +467,7 @@
     if( use_ret == -(MBEDTLS_ERR_ASN1_LENGTH_MISMATCH) )
         mbedtls_snprintf( buf, buflen, "ASN1 - Actual length differs from expected length" );
     if( use_ret == -(MBEDTLS_ERR_ASN1_INVALID_DATA) )
-        mbedtls_snprintf( buf, buflen, "ASN1 - Data is invalid. (not used)" );
+        mbedtls_snprintf( buf, buflen, "ASN1 - Data is invalid" );
     if( use_ret == -(MBEDTLS_ERR_ASN1_ALLOC_FAILED) )
         mbedtls_snprintf( buf, buflen, "ASN1 - Memory allocation failed" );
     if( use_ret == -(MBEDTLS_ERR_ASN1_BUF_TOO_SMALL) )