Add check for platform mesh & texture

It simply checks if the file exists

resources/definitions/cubicon_3dp_110f.def.json

@@ -8,7 +8,6 @@
         "manufacturer": "Cubicon",
         "visible": true,
         "file_formats": "text/x-gcode",
-        "platform": "",
         "supports_usb_connection": false,
         "machine_extruder_trains": {
             "0": "cubicon_3dp_110f_extruder_0"

resources/definitions/cubicon_3dp_210f.def.json

@@ -8,7 +8,6 @@
         "manufacturer": "Cubicon",
         "visible": true,
         "file_formats": "text/x-gcode",
-        "platform": "",
         "supports_usb_connection": false,
         "machine_extruder_trains": {
             "0": "cubicon_3dp_210f_extruder_0"

resources/definitions/cubicon_3dp_310f.def.json

@@ -8,7 +8,6 @@
         "manufacturer": "Cubicon",
         "visible": true,
         "file_formats": "text/x-gcode",
-        "platform": "",
         "supports_usb_connection": false,
         "machine_extruder_trains": {
             "0": "cubicon_3dp_310f_extruder_0"

resources/definitions/delta_go.def.json

@@ -8,7 +8,6 @@
         "manufacturer": "Deltaprintr",
         "file_formats": "text/x-gcode",
         "platform_offset": [0, 0, 0],
-        "platform": "",
         "machine_extruder_trains":
         {
             "0": "delta_go_extruder_0"

resources/definitions/printrbot_play_heated.def.json

@@ -7,7 +7,6 @@
         "author": "Chris Pearson",
         "manufacturer": "Printrbot",
         "file_formats": "text/x-gcode",
-        "platform": "",
         "machine_extruder_trains":
         {
             "0": "printrbot_play_heated_extruder_0"

resources/definitions/ubuild-3d_mr_bot_280.def.json

@@ -9,7 +9,6 @@
         "manufacturer": "uBuild-3D",
         "category": "Other",
         "file_formats": "text/x-gcode",
-        "platform": "mr_bot_280_platform.stl",
         "has_materials": true,
         "preferred_quality_type": "draft",
         "machine_extruder_trains":