This is related to bug #96396. libpllo.so and pluginapp.bin link against GTK+2. That may be by design if the plugin process is meant to load NPAPI plugins that link against GTK+2 as well (e.g. Adobe Flash). However if that's not the case, it may be possible to do something about this. Also note that NPAPI plugins are being deprecated... but I don't really know what LO is doing with NPAPI plugins.
This is by design, and it seems it is there specifically to support Flash. As for the future of NPAPI, there was some discussion last year already: http://lists.freedesktop.org/archives/libreoffice/2014-June/061635.html .
Rene Engelhard committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=4c18af27bf95b332ee2006cfc0bbf469fb1a84d4 tdf#96398 kill remaining NPAPI support for plugins _inside_ LO It will be available in 5.2.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
Thorsten Behrens committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=42645fe67cac22e9a96bc4b34c4f85c09185d9d2 related tdf#96398 pull out more NPAPI-related code It will be available in 5.2.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
Gabor Kelemen committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/help/commit/?id=09d83720ede6138a465e5531ae4dc8feb2bb39a0 related tdf#96398 Remove help for Insert - Object - Plugin
Stephan Bergmann committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=b5589970125ae4b8c0a091cfc12d058ca42ce2ab [API CHANGE] Remove tdf#96398 leftover XRemoteServiceManagerProvider It will be available in 5.3.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.