Turn off the extended resize handle/black triangle when there are no borders

Summary: As discussed and agreed to in T8707: Window borders, 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