fix(lua): don't mutate opts parameter of vim.keymap.del (#18227)

`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`. This is wrong and also undocumented.

(cherry picked from commit 51f33ae5c0)

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