This functionality seems to only have been used with NPAPI plugins.
They've been previously removed.  Per the help text, if it is still
used by anything, you can use the context menu instead.

MN_READONLY_PLUGINOFF also appeared to be unused, so removed.

More code might be removable, but would affect activate on click/visible.

Change-Id: I12d5bf54edd3a2f716912179b87e798b47b3cc3d
Reviewed-on: https://gerrit.libreoffice.org/24500Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>