After I faced this issue, I browsed through discussions on similar topics from past to present to find a solution, but in vain.
I never resorted to options like editing Windows Registry or deleting iTunes temp files.
But I finally found a straight forward way after fiddling through iTunes menus and options (starting a new discussion because others were mostly archived / reply options were not found):
1. Select the Smart Playlist from where you want to delete tracks.
2. Right click on playlist name from left panel.
3. Select Edit Smart Playlist.
4. Uncheck Live Updating at bottom left corner of the popped up window.
5. Close the popup.
6. Now you should be able to delete the tracks either by right click option or from Edit menu.
This has even made the delete option enabled in custom Playlist(s) which had the same song links in the Smart Playlist(s).
This also enabled swipe and delete option on my iPhone and iPad after a fresh Sync (which all were not working before).
[iTunes 18.104.22.168 on Windows 7 Ultimate]
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