[dev-qt/qtwidgets:5] Remove accessible plugin from ebuild

Upstream has made the accessibility plugin part of the normal widgets
code, instead of being a separate plugin. This fixes build failures
stating that "src/plugins/accessible" cannot be found when building with
upstream commit eaee2bd0.

Note that the git tree dev-qt/qtwidgets-5.3.9999 builds does NOT include this commit, so that ebuild does not need to be changed.