Cleanup ttk plugin cmake install logic

[mwestphal]

• Since only the singlePlugin logic is supported now, simplyify the cmake logic
• Remove a duplicate installation of the plugin in the bin directory

[julien-tierny]

hi @mwestphal
thanks a lot for your contribution!
however, it seems that this PR is breaking the continuous integration (see for instance https://github.com/topology-tool-kit/ttk/actions/runs/4668519453/jobs/8265713183?pr=919)

[julien-tierny]

Also, I had a quick look into this and on my systems, the plugin seems to be installed only once (here in /usr/local/bin/plugins/, which I believe is the default path where ParaView will search for plugins, if ParaView was installed in /usr/local). So I'm not so sure about the double install remark. Could you please clarify? Thanks!

[julien-tierny]

Agreed with these two lines.

[mwestphal]

I definitelly misunderstood something but I dont know what yet. However it looks like there multiple install cmake commands where there should be only one or none at all if handled correctly by paraview plugin mechanism.

here in /usr/local/bin/plugins/

Expected location is INSTALL_PREFIX/lib/paraview-5.11/plugins/ not bin.

The reason why I'm trying to do that is that in the paraview superbuild, ttk is installed in bin (and lib) but it should be only in lib.

I need to spend a bit more time on this and understand what it is supposed to do before I can try to fix it.