[doc] kill dtx

The file ``luaotfload.dtx`` which made the documentation a huge PITA to
extend, has finally been converted to ordinary Latex format. In the
course of this conversion the style directives have been moved to file
(``luaotfload-latex.tex``) so as to separate them from the content
(``luaotfload-main.tex``).
[doc,*] adapt makefiles to new doc source