Gather logic to clear excess lines into `s:BuildBufferList()`.

`s:ToggleOnlyOneTab()` lacked the optional parameter to
`s:RebuildBufferList()` required to remove excess lines when the number
of displayed buffers is expected to decrease.  Rather than add this
logic, remove the optional argument from `s:RebuildBufferList()` and
adjust `s:BuildBufferList()` to remove any excess lines in the event
that the line count is too large.  This unburdens callers of
`s:RebuildBufferList()` from the responsibility of determining how any
changes they've made will impact the number of lines in the buffer list.

plugin/bufexplorer.vim

@@ -808,6 +808,11 @@ function! s:BuildBufferList()
 
     let lines = s:MakeLines(table)
     call setline(s:firstBufferLine, lines)
+    let firstMissingLine = s:firstBufferLine + len(lines)
+    if line('$') >= firstMissingLine
+        " Clear excess lines starting with `firstMissingLine`.
+        execute "silent keepjumps ".firstMissingLine.',$d _'
+    endif
     call s:SortListing()
 endfunction
 
@@ -1095,7 +1100,7 @@ endfunction
 " ToggleShowTabBuffer {{{2
 function! s:ToggleShowTabBuffer()
     let g:bufExplorerShowTabBuffer = !g:bufExplorerShowTabBuffer
-    call s:RebuildBufferList(g:bufExplorerShowTabBuffer)
+    call s:RebuildBufferList()
     call s:UpdateHelpStatus()
 endfunction
 
@@ -1109,7 +1114,7 @@ endfunction
 " ToggleShowUnlisted {{{2
 function! s:ToggleShowUnlisted()
     let g:bufExplorerShowUnlisted = !g:bufExplorerShowUnlisted
-    let num_bufs = s:RebuildBufferList(g:bufExplorerShowUnlisted == 0)
+    let num_bufs = s:RebuildBufferList()
     call s:UpdateHelpStatus()
 endfunction
 
@@ -1120,16 +1125,11 @@ function! s:ToggleFindActive()
 endfunction
 
 " RebuildBufferList {{{2
-function! s:RebuildBufferList(...)
+function! s:RebuildBufferList()
     setlocal modifiable
 
     let curPos = getpos('.')
 
-    if a:0 && a:000[0] && (line('$') >= s:firstBufferLine)
-        " Clear the list first.
-        execute "silent keepjumps ".s:firstBufferLine.',$d _'
-    endif
-
     let num_bufs = s:BuildBufferList()
 
     call setpos('.', curPos)