Fix compiler warning for -Wdeprecated-redundant-constexpr-static-def

[ma8ma]

C++17からinline変数が導入されたことによりクラスのconstepxr staticメンバー変数のout-of-line定義が重複になり廃止予定になったとコンパイラーに指摘されたため取り除きます。

clang-17のレポート

src/skeleton/dragtreeview.cpp:35:37: warning: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Wdeprecated-redundant-constexpr-static-def]
   35 | constexpr const char* DragTreeView::s_css_classname;
      |                                     ^
src/skeleton/editview.cpp:783:33: warning: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Wdeprecated-redundant-constexpr-static-def]
  783 | constexpr const char* EditView::s_css_classname;
      |                                 ^
src/skeleton/toolbar.cpp:33:32: warning: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Wdeprecated-redundant-constexpr-static-def]
   33 | constexpr const char* ToolBar::s_css_label;
      |                                ^
src/skeleton/window.cpp:49:33: warning: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Wdeprecated-redundant-constexpr-static-def]
   49 | constexpr const char* JDWindow::s_css_stat_label;
      |                                 ^