tidy: Remove an ineffective cast.

Clang-tidy pointed out a location where a 32-bit integer is being cast
to a 64bit size_t.  This cast is unnecessary and will be done
implicitly by the compiler.

https://clang.llvm.org/extra/clang-tidy/checks/bugprone-misplaced-widening-cast.html

mythtv/libs/libmythtv/mythdeinterlacer.cpp

@@ -146,7 +146,7 @@ void MythDeinterlacer::Filter(VideoFrame *Frame, FrameScanType Scan)
         if (!m_bobFrame->buf || (m_bobFrame->size != Frame->size))
         {
             av_free(m_bobFrame->buf);
-            m_bobFrame->buf = static_cast<unsigned char*>(av_malloc(static_cast<size_t>(Frame->size + 64)));
+            m_bobFrame->buf = static_cast<unsigned char*>(av_malloc(Frame->size + 64));
             m_bobFrame->size = Frame->size;
         }