DiscIO: Centralize the banner pointer and validity boolean into IBannerLoader

These are both used within the banner loaders, and IsValid is exactly the
same. So this makes sense.

Source/Core/DiscIO/BannerLoaderWii.cpp

@@ -21,8 +21,6 @@ namespace DiscIO
 {
 
 CBannerLoaderWii::CBannerLoaderWii(DiscIO::IVolume *pVolume)
-	: m_pBannerFile(nullptr)
-	, m_IsValid(false)
 {
 	u64 TitleID;
 	pVolume->GetTitleID((u8*)&TitleID);
@@ -95,11 +93,6 @@ CBannerLoaderWii::~CBannerLoaderWii()
 	}
 }
 
-bool CBannerLoaderWii::IsValid()
-{
-	return m_IsValid;
-}
-
 std::vector<u32> CBannerLoaderWii::GetBanner(int* pWidth, int* pHeight)
 {
 	SWiiBanner* pBanner = (SWiiBanner*)m_pBannerFile;