Remove custom content item from ActionMenu separator

Summary:
Since MenuSeparator is properly styled by QQC2 Desktop Style these days,
there is no need for custom styling here. In addition, it actually leads
to incorrect results.

Before:

After:

Test Plan:
Run a Kirigami application with a Menu with a separator. Observe that it
works correctly.

Reviewers: Kirigami, mart, ngraham

Reviewed By: Kirigami, ngraham

Subscribers: apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D23784