Bug 675391 - Remove support for EVOLUTION_PLUGIN_PATH.

This was a hidden feature to load EPlugins from a custom location by
setting the EVOLUTION_PLUGIN_PATH environment variable.  It defaulted
to $HOME/.eplugins, which is in violation of the XDG Base Directory
Specification.

Since I've never ever heard of anyone using this and wasn't even aware
of it myself, rather than migrating the folder to a standard-compliant
location I'm just going to drop support for EVOLUTION_PLUGIN_PATH and
put my money on no one noticing.  The EPlugin framework is gradually
being decommissioned anyway in favor of the simpler and more flexible
EExtension framework in Evolution-Data-Server.