[MC][NFC] Drop unused MCDecodedPseudoProbeInlineTree::ChildrenToProcess (#100576)

The usage was removed in 3f97016.

Results in a slight peak RSS reduction in
`perf2bolt --profile-use-pseudo-probes` from 17.24 to 16.85 GiB.

Author: aaupov

llvm/include/llvm/MC/MCPseudoProbe.h

@@ -280,8 +280,6 @@ class MCDecodedPseudoProbeInlineTree
                                          MCDecodedPseudoProbeInlineTree> {
 public:
   InlineSite ISite;
-  // Used for decoding
-  uint32_t ChildrenToProcess = 0;
 
   MCDecodedPseudoProbeInlineTree() = default;
   MCDecodedPseudoProbeInlineTree(const InlineSite &Site) : ISite(Site){};