Rename PLYReader to TrimeshReader

We're going to repurpose this plug-in to read other file formats as well.
Currently it still only registers itself as PLY file reader. But I'm about to change that.

Contributes to issue CURA-6739.

plugins/PLYReader/__init__.py

@@ -1,21 +0,0 @@
-# Copyright (c) 2019 Ultimaker
-# Cura is released under the terms of the LGPLv3 or higher.
-
-from . import PLYReader
-
-from UM.i18n import i18nCatalog
-i18n_catalog = i18nCatalog("uranium")
-
-
-def getMetaData():
-    return {
-        "mesh_reader": [
-            {
-                "extension": "ply",
-                "description": i18n_catalog.i18nc("@item:inlistbox", "PLY File")
-            }
-        ]
-    }
-
-def register(app):
-    return {"mesh_reader": PLYReader.PLYReader()}