STYLE: Remove include guard from elastixlib.cxx
STYLE: Remove include guard from elastixlib.cxx

Removed the `__elastixlib_cxx` include guard, as was already mentioned as "todo".

Also did a cleanup of the #include statements in elastixlib.cxx.

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix