[processing] do not declare output of saveselectedfeatures as points

[volaya]

Description

Algorithm output can be of any geometry type

Checklist

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[alexbruy]

Not related to this PR, but I'm wondering why we use QgsProcessingParameterVectorLayer here instead of feature source which is more flexible?

[volaya]

Looks like it was directly adapted from the old QGIS2 code, and that change was overlooked

[nyalldawson]

It's because it works directly with QgsVectorLayer methods -- i.e. directly accesses the selected features from the layer. Feature source parameters themselves have no knowledge of selections, they just get a bunch of features and it's the parameter value which determines whether these are the selected features only.