[FEFF/FDMNES input generation] remove `struct2xas`/`structure2feff` and keep Larixite only

[maurov]

The deprecated modules larch.xrd.structure2feff and larch.xrd.stuct2xas should be removed from Larch, in favor of Larixite. This task is in pair with Larixite#4.

[maurov]

@newville do you agree that I remove such modules by next release and we keep Larixite only? I have not fully checked, but Larix GUI is using only Larixite, right? I will update the notebooks accordingly.

[newville]

@maurov Yes. Let's remove these.