Remove duplicate <main> tags in management views

There can only be one <main> tag in a document, and we've already got a
<main> tag in the management layout.

app/views/management/budgets/investments/index.html.erb

@@ -1,4 +1,3 @@
-<main>
 <span class="not-print">
   <%= render "admin/shared/budget_investment_search", url: management_budget_investments_path(@budget) %>
 </span>
@@ -27,4 +26,3 @@
     <%= paginate @investments %>
   </div>
 </div>
-</main>

app/views/management/budgets/investments/print.html.erb

@@ -1,4 +1,3 @@
-<main>
 <span class="not-print">
   <%= render "admin/shared/budget_investment_search", url: print_management_budget_investments_path(@budget) %>
 </span>
@@ -34,4 +33,3 @@
     </div>
   </div>
 </div>
-</main>

app/views/management/proposals/index.html.erb

@@ -1,4 +1,3 @@
-<main>
 <h2><%= t("management.proposals.index.title") %></h2>
 
 <%= render Admin::SearchComponent.new(label: t("admin.shared.search.label.proposals")) %>
@@ -21,4 +20,3 @@
     </div>
   </div>
 </div>
-</main>

app/views/management/proposals/print.html.erb

@@ -1,4 +1,3 @@
-<main>
 <div class="row">
   <div class="proposals-list">
     <a id="print_link" href="javascript:window.print();" class="button warning float-right">
@@ -26,4 +25,3 @@
     </div>
   </div>
 </div>
-</main>