[InlineMessage] Do not draw shadows around the message

Summary:
This patch removes InlineMessage's DropShadow for the following reasons:

1. consistency - there are no shadows in QWidgets' inline messages
2. visual hierarchy - shadows add depth, making the message seem like it is floating over the window's content and thereby also making the message seem fairly prominent
3. aesthetics - no shadows means a cleaner look

Test Plan:
Before:

Reviewers: VDG, Kirigami, ngraham, mart

Reviewed By: VDG, Kirigami, ngraham, mart

Subscribers: Codezela, rooty, ndavis, mart, abetts, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D19044