Drop plugin framework out of the current architecture.

[hsfzxjy]

According to a series of research, there is no perfect solution to support a plugin framework. Stability contrasts with hot-reloadability, and varied utilities contrasts with security. On the other hand, our server with limited resources cannot support plugins uploaded from even tens of users. The plugin framework prevents the core part of the project from scaling up.

But this doesn't mean that Biohub will change to be conservative. There are multiple ways to help the community, and integrating plugins is just one of them. In the new version, we may open up some interfaces for public data accessing.