Remove unused variable script_dir

This variable used to be set based on either $HOME or $script_path. However,
$script_path is no longer set, so when $HOME is unset there will be an
'dirname: missing operand' error message. $script_dir is no longer used
either, so we can safely remove it entirely.
Merge pull request #75 from raboof/remove_unused_variable_mergable

Remove unused variable script_dir