MuonTrackExtrapolator - Remove second extrapolator instance

Johannes Junggeburthrequested to merge
jojungge/athena:Extrapolator2 into 22.0

Hi,

this MR removes the m_extrapolator2 & cleans up a bit the code using smart pointers

Closes ATLASRECTS-6658

Edited by Johannes Junggeburth

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