Fix obsolete mention of non-int64 support in CREATE SEQUENCE documentation.

The old text explained what happened if we didn't have working int64
arithmetic.  Since that case has been explicitly rejected by configure
since 8.4.3, documenting it in the 9.x branches can only produce confusion.

doc/src/sgml/ref/create_sequence.sgml

@@ -224,10 +224,7 @@ SELECT * FROM <replaceable>name</replaceable>;
   <para>
    Sequences are based on <type>bigint</> arithmetic, so the range
    cannot exceed the range of an eight-byte integer
-   (-9223372036854775808 to 9223372036854775807).  On some older
-   platforms, there might be no compiler support for eight-byte
-   integers, in which case sequences use regular <type>integer</>
-   arithmetic (range -2147483648 to +2147483647).
+   (-9223372036854775808 to 9223372036854775807).
   </para>
 
   <para>