Manage variables related to both LaTeX export and snippets

Manage variables related to both LaTeX export and snippets

* lisp/org.el (org-format-latex-header-extra, org-export-have-math):
  Removed variables.
(org-latex-default-packages-alist): Renamed from
`org-export-latex-default-packages-alist'.
(org-latex-packages-alist): Renamed from
`org-export-latex-packages-alist'.
(org-try-cdlatex-tab, org-cdlatex-underscore-caret,
org-cdlatex-math-modify): Reorder in file.
(org-format-latex): Remove `org-format-latex-header-extra'.
(org-create-formula-image-with-dvipng,
org-create-formula-image-with-imagemagick): Apply variables renaming
and removal.
* lisp/org-entities.el (org-entities-user): Update docstring.
* lisp/ox-latex.el (org-latex-classes, org-latex-listings): Update
  docstring.
(org-latex-guess-inputenc): Renamed from `org-latex--guess-inputenc'.
(org-latex-guess-babel-language): Renamed from
`org-latex--guess-babel-language'.
(org-latex-template): Apply renaming.
* lisp/ox-beamer.el (org-beamer-template): Apply renaming.
* lisp/ob-latex.el (org-babel-execute:latex): Apply variable renaming
  and removal.
(org-babel-latex-tex-to-pdf): Call `org-latex-compile' instead of
copying it.