Update EGamma conversion flags

The configurations relevant for the reconstruction of photon conversions from tracks are maintained by the EGamma group and as such it seems more natural that the relevant SecVertex flags live under the Egamma flag (pun intended).

Consequently, this MR:

• remove the unused set createSecVertexingFlags and only preserve createEGammaSecPileUpVertexingFlags
• move the corresponding flags from Tracking to Egamma
• clean up a bit some flags for which the values are unlikely to be changed (the mass of the electron, dummy values...)

Checked to leave AOD and DAOD_PHYSVAL outputs unchanged for Run 3 config.

FYI @christos @jdevivi