Remove undocumented `m` mapping used for debugging old MRU algorithm.

plugin/bufexplorer.vim

@@ -833,7 +833,6 @@ function! s:MapKeys()
     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> m             :call <SID>MRUListShow()<CR>
     nnoremap <script> <silent> <nowait> <buffer> o             :call <SID>SelectBuffer()<CR>
     nnoremap <script> <silent> <nowait> <buffer> p             :call <SID>ToggleSplitOutPathName()<CR>
     nnoremap <script> <silent> <nowait> <buffer> q             :call <SID>Close()<CR>