add_filter('category_rewrite_rules', function($rules) {
foreach($rules as $key => $val) {
if(strpos($key, 'category/') === 0) {
unset($rules[$key]);
}
}
return $rules;
});

add_filter('request', function($query_vars) {
if(isset($query_vars['category_name'])) {
$query_vars['category_name'] = str_replace('category/', '', $query_vars['category_name']);
}
return $query_vars;
});