LibGUI: Remove public getter `Button::is_mimic_pressed()`

This method shouldn't be used outside the class and having a getter for
a bool seems a bit overkill.

Userland/Libraries/LibGUI/Button.cpp

@@ -57,7 +57,7 @@ void Button::paint_event(PaintEvent& event)
     Painter painter(*this);
     painter.add_clip_rect(event.rect());
 
-    bool paint_pressed = is_being_pressed() || is_mimic_pressed() || (m_menu && m_menu->is_visible());
+    bool paint_pressed = is_being_pressed() || m_mimic_pressed || (m_menu && m_menu->is_visible());
 
     Gfx::StylePainter::paint_button(painter, rect(), palette(), m_button_style, paint_pressed, is_hovered(), is_checked(), is_enabled(), is_focused(), is_default() && !another_button_has_focus());
 
@@ -260,7 +260,7 @@ void Button::set_mimic_pressed(bool mimic_pressed)
 
 void Button::timer_event(Core::TimerEvent&)
 {
-    if (is_mimic_pressed()) {
+    if (m_mimic_pressed) {
         m_mimic_pressed = false;
 
         update();

Userland/Libraries/LibGUI/Button.h

@@ -64,7 +64,6 @@ public:
     bool another_button_has_focus() const { return m_another_button_has_focus; }
 
     void set_mimic_pressed(bool mimic_pressed);
-    bool is_mimic_pressed() const { return m_mimic_pressed; };
 
     virtual Optional<UISize> calculated_min_size() const override;