Remove the need for a non-filetype plugin

Everything that was set in plugin/jedi.vim can be safely set in
ftplugin/python/jedi.vim instead, thus obviating the need for global
initialization. Additionally, because no global changes are made, the
check for g:loaded_jedi can be omitted as well.

Apart from that, all functionality from plugin/jedi.vim has simply been
copied over into ftplugin/python/jedi.vim.
Remove duplicate omnifunc setting