get rid of encoding argument

[jannes-m]

This addresses and solves #218.
qgis_run() calls processx::run().
In the documentation of the latter it says on the encoding argument:

The encoding to assume for stdout and stderr. By default the encoding of the current locale is used. Note that processx always reencodes the output of both streams in UTF-8 currently.

Hence, it seems there is no need to explicitly set the encoding.
What do you think @florisvdh?
Or am I overlooking any unwanted side-effects?
FYI: devtools::check() also runs successfully with this modification.

[florisvdh]

Thanks @jannes-m for addressing this. We should give this a try; if it poses no further issues or side effects this would definitely be a simplification on the {qgisprocess} side. For consistency, I propose to add PR #220 in this PR.

I could not find any consequences for now; based on r-lib/processx#265 and r-lib/processx#299 I assume indeed this should work.