Fix/validates length of min max length

https://www.openproject.org/work_packages/14632
This is nit picking. But you don't need to set the `max_size` value. The `if`-statement already returns a value. So, you could shorten the method like this:

````ruby
def allowed_size_of_integer
  size_in_byte = CustomField.columns.find { |c| c.name == 'min_length' }.limit
  if size_in_byte.present?
    2**(8*size_in_byte)/2 - 1
  else
    #Postgres does not set the limit but has a limit of 4 bytes
    2147483647
  end
end
````