Switch to native vim plugin management

Since the vim plugins are stored in submodules and vim-plug is not a big
fan of that when installing (Needs PlugClean and PlugInstall again
although the repos of the plugins were already pulled) it makes more
sense to me to use vim8s native plugin management.

.config/vim/vimrc

@@ -1,15 +1,12 @@
 " Plugins ######################################################################
-call plug#begin('~/.config/vim/plugged')
-
 " Auto completion
-Plug 'neoclide/coc.nvim', {'branch': 'release'}
+packadd! coc.nvim
 source $XDG_CONFIG_HOME/vim/coc.nvim.vim
 " Theme
-Plug 'joshdick/onedark.vim'
+packadd! onedark.vim
 " Git information
-Plug 'airblade/vim-gitgutter'
+packadd! vim-gitgutter
 
-call plug#end()
 " Keybinds #####################################################################
 " Set leader key
 let mapleader = "\<Space>"