Remove SelectionList::update_timestamp()

This single line member function was only used once, inline it
directly.

src/selection.cc

@@ -48,12 +48,11 @@ void SelectionList::set(Vector<Selection> list, size_t main)
     kak_assert(main < list.size());
     m_selections = std::move(list);
     m_main = main;
+    m_timestamp = m_buffer->timestamp();
     sort_and_merge_overlapping();
-    update_timestamp();
     check_invariant();
 }
 
-
 namespace
 {
 

src/selection.hh

@@ -137,7 +137,6 @@ struct SelectionList
     Buffer& buffer() const { return *m_buffer; }
 
     size_t timestamp() const { return m_timestamp; }
-    void update_timestamp() { m_timestamp = m_buffer->timestamp(); }
 
     void insert(ConstArrayView<String> strings, InsertMode mode,
                 Vector<BufferCoord>* out_insert_pos = nullptr);