Unfortunately there is no way to permanently remove "Safe Mode" as a feature from the phone. It is actually needed.
There are a few ways to get out of it, however, they will only work if your buttons are also functioning correctly. If you say, turn the phone off and then back on to get out of that mode, if your button is still stuck/not working correctly, it will most likely put you right back into "Safe Mode".
You can try, while the phone is on, and use the "Status Bar" to turn off "Safe Mode'. Pull down (swipe) the "Status Bar" of your phone. Now tap the "Safe Mode" button. This should turn "Safe Mode" off. Now your phone will automatically restart, so that it boots up out of "Safe Mode".
However, it is likely, again, that it will go right back into "Safe Mode" if the button is a problem.
The only sure way to get out of it and stay out of it requires you have the button fixed.