Remove useless range checks on INT8 sequences

Remove useless range checks on INT8 sequences

There's no point in checking if an INT8 sequence has a seqmin and seqmax
value is outside the range of the minimum and maximum values for an int64
type.  These both use the same underlying types so an INT8 certainly
cannot be outside the minimum and maximum values supported by int64.

This code is fairly harmless and it seems likely that most compilers
would optimize it out anyway, never-the-less, let's remove it replacing
it with a small comment to mention why the check is not needed.

Author: Greg Nancarrow, with the comment revised by David Rowley
Discussion: https://postgr.es/m/CAJcOf-c9KBUZ8ow_6e%3DWSfbbEyTKfqV%3DVwoFuODQVYMySHtusw%40mail.gmail.com