LibGfx: Remove unused current_system_theme() function

Userland/Libraries/LibGfx/SystemTheme.cpp

@@ -14,12 +14,6 @@ static SystemTheme dummy_theme;
 static const SystemTheme* theme_page = &dummy_theme;
 static Core::AnonymousBuffer theme_buffer;
 
-const SystemTheme& current_system_theme()
-{
-    VERIFY(theme_page);
-    return *theme_page;
-}
-
 Core::AnonymousBuffer& current_system_theme_buffer()
 {
     VERIFY(theme_buffer.is_valid());

Userland/Libraries/LibGfx/SystemTheme.h

@@ -140,7 +140,6 @@ struct SystemTheme {
     char path[(int)PathRole::__Count][256]; // TODO: PATH_MAX?
 };
 
-const SystemTheme& current_system_theme();
 Core::AnonymousBuffer& current_system_theme_buffer();
 void set_system_theme(Core::AnonymousBuffer);
 Core::AnonymousBuffer load_system_theme(const String& path);