Revert "LaTeX reader: citation handling changes."

This reverts commit f7229b147314042f946dfded3b441ab0fae260a0.
Markdown writer:  Omit bibliography when `citations` enabled.

In 1.11, citations would be rendered as pandoc markdown citations,
but the bibliography would still be printed.

We avoid that by adding a `RawBlock "pandoc" "references"` before
the references.  This allows the markdown writer to find the references
and strip them off when `citations` is enabled.
Updated changelog.
Markdown writer:  New approach for citations.

* Reverts 1.11 change that caused citations to be rendered as
  markdown citations, even if `--biblio` was specified, unless
  `citation` extension is disabled.  Now, formatted citations
  are always printed if `--biblio` was specified.  If you want to
  reformat markdown keeping pandoc markdown citations intact,
  just don't specify `--biblio`.

* Reverted now unnecessary changes to Text.Pandoc.Biblio adding the raw
  block to mark the bibliography, and to Text.Pandoc.Writers.Markdown
  to remove the bibliography if `citations` not specified.

* If the content of a `Cite` inline is a `RawInline "latex"`, which
  means that a LaTeX citation command was parsed and `--biblio` wasn't
  specified, then render it as a pandoc markdown citation.  This means
  that `pandoc -f latex -t markdown`, without `--biblio`, will convert
  LaTeX citation commands to pandoc markdown citations.
Updated changelog.