KDE remove title bar and borders on maximized windows

The method is as follows:

Use you favorite text editor to edit the file

~/.kde4/share/config/kwinrc

In some distributions the .kde4 directory is called .kde (or something else like .kdemod4). In Plasma 5, it's at ~/.config/kwinrc.

Add the line

BorderlessMaximizedWindows=true

under the section [Windows] It should look something like this:

[Windows]
ActiveMouseScreen=true
AltTabStyle=KDE
AutoRaise=false
AutoRaiseInterval=750
BorderSnapZone=10
BorderlessMaximizedWindows=true
CenterSnapZone=0

Launch KRunner (Alt+F2) and run the command

kwin --replace

or in Plasma 5

kwin_x11 --replace

to restart KWin

Go crazy and maximize every window in sight! Remember that you can unmaximize them from the window menu (Alt+F3) or from the task manager in your panel. If quick tiling is enabled (System Settings → Desktop → Screen Edges → Window Management), you can also use Alt+Left mouse button to unmaximize windows by dragging them.

This answer was copied from here and not my original writing.