on the search page we can see for ex. the location "Ηράκλειο, GR" to be shown up inside the <div class="sub-title">... </div>

how can we remove the ", GR" ?

Open search.php, check what functions generates it.
Then go to functions.php, find that function and remove country code.

ok, its the "<?php echo $location; ?>"

@developGr
$location will be populated by some function. That function will be created in functions.php.