Take a look at the communities post titled How to change default tab to internal comment.
In addition, one of the comments mentions a third party add-on called Doublecheck for JIRA Service Desk that may be able to assist with this as well.
Finally, take a look at Respond to Customer and Internal Comment tabs missing from comment in Service Desk. While this article aims to restore functionality you may want to go the other way and disable the service-desk-comment-field module. I disabled that module and I'm left with the following:
Hopefully that helps.
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