AK: Remove unnecessary casts to size_t, after Vector changes

Now that Vector uses size_t, we can remove a whole bunch of redundant
casts to size_t.

AK/StringView.cpp

@@ -40,7 +40,7 @@ StringView::StringView(const String& string)
 
 StringView::StringView(const ByteBuffer& buffer)
     : m_characters((const char*)buffer.data())
-    , m_length((size_t)buffer.size())
+    , m_length(buffer.size())
 {
 }