Don't pass through macro definitions themselves when `latex_macros`
is set. The macros have already been applied.
If `latex_macros` is enabled, then `rawLaTeXBlock` in
Text.Pandoc.Readers.LaTeX will succeed in parsing a macro definition,
and will update pandoc's internal macro map accordingly, but the
empty string will be returned.
Together with earlier changes, this closes#4179.