This is a built-in part of Jira Service Management cloud. Are you referring to the Create major incident or the Link issue button?
If you don't want to see the Create major incident button you could try using a different Jira Service Management template. Right now, that button will only show in projects created with the new IT Service Management template. If you create the project using a different template you won't see that button.
If you're referring to the Link issue button there is no way to get rid of that.
Finally, can I ask, why do you want to remove either of the buttons?
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