Remove "Draw separator under active window's titlebar" windeco option

This option is off by default and has been for several years, after it was briefly turned on by default and proved to be unpopular with users, generating many complaints and bug reports. The line uses a semantically inappropriate highlight color which is far too visually bright, and it doesn't look good with any of our header-color-using color schemes. Arguably it never looked good with older color schemes where the titlebar and window background use different colors either. Let's remove this option.

BUG: 427422 FIXED-IN: 5.24

cc @teams/vdg