Disable Line Modification Indicators
On KDE 4.8.0, there is no way to disable the line modification markers.
In KDE >= 4.8.1, you can disable them as follows by first closing Kate and then typing
kwriteconfig --file katerc --group "Kate View Defaults" --key "Line Modification" --type bool false
To enable it again, close Kate and run
kwriteconfig --file katerc --group "Kate View Defaults" --key "Line Modification" --type bool true
However, this only affects Kate; not KWrite, Kile, KDevelop or any other application using Kate Part. If you want to disable the markers for KWrite, Kile or KDevelop, use kwriterc, kilerc or kdeveloprc instead of katerc.
In KDE >= 4.9, there is a graphical option in the editor configuration dialog in “Appearance > Borders > [x] Show line modification markers”.