Simplify URL to destroy a booth assignment

We can find the booth through the booth assignment, so we don't need to
pass it in the URL.

Since the parameter is in the URL and not sent through a form, we can
also use `params[:poll_id]` directly, and so we can reuse the
`load_poll` method.

app/controllers/admin/poll/booth_assignments_controller.rb

@@ -1,5 +1,5 @@
 class Admin::Poll::BoothAssignmentsController < Admin::Poll::BaseController
-  before_action :load_poll, except: [:create, :destroy]
+  before_action :load_poll, except: [:create]
 
   def index
     @booth_assignments = @poll.booth_assignments.includes(:booth).order("poll_booths.name")
@@ -36,9 +36,8 @@ class Admin::Poll::BoothAssignmentsController < Admin::Poll::BaseController
   end
 
   def destroy
-    @poll = Poll.find(booth_assignment_params[:poll_id])
-    @booth = Poll::Booth.find(booth_assignment_params[:booth_id])
-    @booth_assignment = ::Poll::BoothAssignment.find(params[:id])
+    @booth_assignment = @poll.booth_assignments.find(params[:id])
+    @booth = @booth_assignment.booth
 
     @booth_assignment.destroy!