fix: don't mutate opts parameter of vim.keymap.del

`vim.keymap.del` takes an `opts` parameter that lets caller refer to and
delete buffer-local mappings. For some reason the implementation of
`vim.keymap.del` mutates the table that is passed in, setting
`opts.buffer` to `nil`. I'm not sure why it does this since `opts` is
never again accessed/used by the implementation anyway.

This seems wrong and is also undocumented. This bit me because I was
setting and then later unsetting a collection of mappings for some
buffer, and I created a local `opts = {buffer=my_buffer_number}`, and
then I got "no such mapping" errors.

runtime/lua/vim/keymap.lua

@@ -130,7 +130,6 @@ function keymap.del(modes, lhs, opts)
   local buffer = false
   if opts.buffer ~= nil then
     buffer = opts.buffer == true and 0 or opts.buffer
-    opts.buffer = nil
   end
 
   if buffer == false then