Fix parse_time() handling of fractional seconds

There's no Token::Period in such situation, so fractional part (from sec) was silently truncated.

Can't uncomment the test yet, because parse_timestamp() is effectively
unused: the code added to parse_value() in 5abd9e7dec
was wrong as it attempted to handle unquoted date/time literals. One
part of it was commented out earlier, the other can't work as far as I
can see, as it tries to parse a Number token - `([0-9]|\.)+` - as a
timestamp, so I removed it as well.

tests/sqlparser_postgres.rs

@@ -657,6 +657,7 @@ fn parse_timestamps_example() {
     let sql = "2016-02-15 09:43:33";
     let _ = parse_sql(sql);
     //TODO add assertion
+    //assert_eq!(sql, ast.to_string());
 }
 
 #[test]
@@ -664,6 +665,7 @@ fn parse_timestamps_with_millis_example() {
     let sql = "2017-11-02 19:15:42.308637";
     let _ = parse_sql(sql);
     //TODO add assertion
+    //assert_eq!(sql, ast.to_string());
 }
 
 #[test]