[IMT] Remove redundant call to EnableImplicitMT
[IMT] Remove redundant call to EnableImplicitMT

EnableParBranchProcessing is only called by TParBranchProcessingRAII's
constructor, which is only called by TTree::GetEntry if
ROOT::IsImplicitMTEnabled returns true. Plus, it's surprising that
a RAII object that is meant to activate locks also changes the state
of ROOT's thread-pool (permanently). As these facilities are in
ROOT::Internal, this change in behavior in TParBranchProcessingRAII
will not be user-visible.

root-mirror/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,olifre/root,karies/root,karies/root,karies/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,olifre/root,olifre/root,olifre/root,root-mirror/root,karies/root,root-mirror/root