Remove Markdown plugin plasticboy/vim-markdown

Removed the plasticboy/vim-markdown (1) plugin (added in GH-38) again.
The default Markdown syntax has been updated and now also supports many
(non-standard) Markdown extensions while the plugin causes some
compatibility problems.

References:

  (1) https://github.com/plasticboy/vim-markdown

Closes GH-129

snowblocks/vim/vimrc

@@ -33,7 +33,6 @@ Plug 'airblade/vim-gitgutter'
 "+--- Syntax ---+
 Plug 'pangloss/vim-javascript'
 Plug 'mxw/vim-jsx'
-Plug 'plasticboy/vim-markdown'
 
 "+--- Color Themes ---+
 Plug 'arcticicestudio/nord-vim'