Yes, it’s definitely Note Expression. For my own benefit - to confirm to myself and to learn something about note expression - I recreated the image by creating arpeggios with sustains and then I ran Midi->NoteExpression->ConvertToNoteExpression on the midi part and it is the same. So, as explanation, see my image below.
Ring 1 - The ShowNoteExpressionData button - show and hide
Ring 2 - In fact the Sustain events are converted into note Release Lengths (and the sustain data isn’t deleted, whereas it is for other controllers)
Ring 3 - I reduced the Release Length on this selected midi note - you can remove the ‘sustain’ tail entirely by setting this to zero
Two ways to answer the OP’s question:
Select all notes and then change Release Length to Zero (double-click in release length so you have all text selected then type 0 and press Ctrl-Enter to set all values absolutely zero (rather than relatively, which is what happens if you don’t press Ctrl))
Hide the Note Expression Data using the show/hide button.