applets/devicenotifier: don't show "Mount" action for MTP devices

Mounting doesn't exist as a concept for these devices.

BUG: 446278 FIXED-IN: 5.27.5

Tested in Plasma 5 with my phone in MTP mode ("Mount" action was gone) and a USB mass storage disk ("Mount" action was still there and worked)


cc @ratijas @sitter

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