Remove visual-mode mappings for BufExplorer window.

These mappings for `d` and `D` may have been intended to support
selecting multiple buffers at once and deleting them as a group; at
present, however, support for this is incomplete.  In addition, it's
difficult to enter visual mode in the normal manner, as both `v` and `V`
have been redefined for other purposes.  CTRL-v is still available for
starting visual block mode, but this is less commonly known.

Support for the use case of "select multiple and delete as a group" has
been requested in issue #4, "Delete multiple buffers removes the wrong
ones"; if that feature is to be provided, it will likely be worked in
the context of that issue.

plugin/bufexplorer.vim

@@ -703,9 +703,7 @@ function! s:MapKeys()
     nnoremap <script> <silent> <nowait> <buffer> b             :call <SID>SelectBuffer("ask")<CR>
     nnoremap <script> <silent> <nowait> <buffer> B             :call <SID>ToggleOnlyOneTab()<CR>
     nnoremap <script> <silent> <nowait> <buffer> d             :call <SID>RemoveBuffer("delete")<CR>
-    xnoremap <script> <silent> <nowait> <buffer> d             :call <SID>RemoveBuffer("delete")<CR>
     nnoremap <script> <silent> <nowait> <buffer> D             :call <SID>RemoveBuffer("wipe")<CR>
-    xnoremap <script> <silent> <nowait> <buffer> D             :call <SID>RemoveBuffer("wipe")<CR>
     nnoremap <script> <silent> <nowait> <buffer> f             :call <SID>SelectBuffer("split", "sb")<CR>
     nnoremap <script> <silent> <nowait> <buffer> F             :call <SID>SelectBuffer("split", "st")<CR>
     nnoremap <script> <silent> <nowait> <buffer> o             :call <SID>SelectBuffer()<CR>