I wouldn't recommend deleting it.
It's there as an alternative to the DOMPDF engine. There's a new option in the Fabrik global settings to choose which PDF rendering engine to use, DOMPDF or MPDF. As you know, PDF rendering is kind of sketchy, we have to rely on 3rd party libraries to do the HTML to PDF rendering, and those two libraries both have "quirks" (a polite word for "bugs"). DOMPDF works better for some things, MPDF works better for others.
Yes, it's huge, but unless 98Mb is causing you significant disk space issues (which these days I very much doubt, I can't remember the last time I saw a client's server with less than a few gig spare), don't worry about it. Removing it could cause issues with the Composer autoloading, as the Composer config expects it to be there, and will tell PHP there's an autoloader for it. I'm not 100% sure, but I think removing the folder could then cause bizarre PHP errors in certain circumstances.
Also, the next time you update Fabrik (either from github, or by updating the Fabrik Library component), it'll come back.
-- hugh