Remove matplotlib from "optional dependencies" docs

matplotlib dep was removed with d35e3f8
under #472

Author: soxofaan

docs/installation.rst

@@ -86,7 +86,6 @@ Depending on your use case, you might also want to install some additional libra
 For example:
 
 - ``netCDF4`` or ``h5netcdf`` for loading and writing NetCDF files (e.g. integrated in ``xarray.load_dataset()``)
-- ``matplotlib`` for visualisation (e.g. integrated plot functionality in ``xarray`` )
 - ``pyarrow`` for (read/write) support of Parquet files
   (e.g. with :py:class:`~openeo.extra.job_management.MultiBackendJobManager`)
 - ``rioxarray`` for GeoTIFF support in the assert helpers from ``openeo.testing.results``