FileManager: don't load already loaded plugins (#6958)
FileManager: don't load already loaded plugins (#6958)

FileManager:init is called by FileManager:reinit, which caused duplicate
loads. Those can't be useful, (for example to re-load a modified plugin
on runtime), as PluginLoader:loadPlugins memoizes loaded plugins.
mwoz123/koreader,Markismus/koreader,NiLuJe/koreader,poire-z/koreader,Hzj-jie/koreader,NiLuJe/koreader,poire-z/koreader,pazos/koreader,koreader/koreader,Frenzie/koreader,koreader/koreader,Frenzie/koreader