You can turn it off, but it can not be removed. RTT (in the default phone call screen) is an accessibility feature.
To disable it in normal voice calls: Open the "Phone" app tap the menu button (three vertical dot icon in the upper right or left corner of the display), scroll until you see "Real Time Text" and open that menu and select "Visible During Calls."
If that doesn't satify you... the only other way to make the button disappear is to use a 3rd party phone app, which doesn't support RTT or allows RTT to be disabled entirely. This is not recommended as using 3rd party phone apps may expose private information or introduce new security vulnerabilities.
