Remove unused legislation proposals map action

The only view that linked to this action was never used and so it was
deleted in commit 0bacd5baf.

Since now the proposals controller is the only one place rendering the
`shared/map` partial, we're moving it to the proposals views.

app/controllers/proposals_controller.rb

@@ -77,6 +77,11 @@ class ProposalsController < ApplicationController
     @tag_cloud = tag_cloud
   end
 
+  def map
+    @proposal = Proposal.new
+    @tag_cloud = tag_cloud
+  end
+
   def disable_recommendations
     if current_user.update(recommended_proposals: false)
       redirect_to proposals_path, notice: t("proposals.index.recommendations.actions.success")