Summary: As discussed and agreed to in {T8707}, turn off the black triangle that's currently drawn by default when windows are drawn with "No Borders"

Test Plan:
- Tested on a new user: no black triangle when KWin's border setting is "No Borders"
- Tested on an existing user who had `DrawSizeGrip=false` in their `breezerc` file: no change
- Tested on an existing user with who had `DrawSizeGrip=false` in their `breezerc` file and then turned it on in System Settings: black triangle appears as expected
- With the black triangle on, reset to default settings: works as expected

Reviewers: #vdg, #breeze, #plasma, abetts, romangg

Reviewed By: #vdg, #plasma, abetts, romangg

Subscribers: hpereiradacosta, romangg, plasma-devel

Tags: #plasma

Maniphest Tasks: T8707

Differential Revision: https://phabricator.kde.org/D13277