Fix scaling ITS CA tracker params for low Bfield

[shahor02]

The problem of missing ITS tracks in the low B-field run was due to the excessive application (in the unrelated loop) of the scaling factor <1 (real_field/5kG) to param.TrackletMinPt, leading to param.TrackletMinPt~0.
This PR fixes it by applying the factor only once, leading to improved reconstruction (compared to no scaling, red and blue on the plot below).

@mpuccio still, it is not clear why setting the param.TrackletMinPt close to 0 leads to the loss of tracks. I guess it is related to lines

AliceO2/Detectors/ITSMFT/ITS/tracking/src/TimeFrame.cxx

Lines 419 to 421 in a8f7574

	float oneOverR{0.001f * 0.3f * std::abs(mBz) / trkParam.TrackletMinPt};
	for (unsigned int iLayer{0}; iLayer < mClusters.size(); ++iLayer) {
	mMSangles[iLayer] = MSangle(0.14f, trkParam.TrackletMinPt, trkParam.LayerxX0[iLayer]);

, but not sure why.

[github-actions]

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