Remove border around menubars

Summary:
Before
{F5800988, layout=center, size=full}

After
{F5800989, layout=center, size=full}

Before(dark theme)
{F5800990, layout=center, size=full}

After(dark theme)
{F5800991, layout=center, size=full}

Reviewers: #breeze, ngraham, jackg, #vdg

Reviewed By: #breeze, ngraham, #vdg

Subscribers: abetts, ndavis, filipf, jackg, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12055