How to remove some permissions from an app?

Yes but, if I’m not mistaken, not only.
edit: well, I was mistaken. See next post.
It seems possible to remove/add permissions by modifying two files:

The one you mention:
>/usr/share/applications/sailfish-browser.desktop

and
>/home/.system/var/lib/sailjail/settings/user-100000.settings

Permissions for a given app have to match in both files.

@headlamp_mapper, think of making a backup of these files before editing them.
About the easiest way to edit them, I don’t know exactly. I use vim as root.
Is there a simpler way, btw?
You can of course also replace strings with sed, if you know it, as advised by @mive.

You also can use the search function of the forum to find more info, for example: