[GeoMechanicsApplication] Remove GeoStructural Elements where justified + associated impact

[indigocoral]

Depends on the investigation task: [GeoMechanicsApplication] Investigate the Structural Elements in GeoMechanics

[avdg81]

When the last beam element class (GeoCrBeamElementLinear2D2N) is removed from GeoMechanicsApplication, we should also remove variable RESET_DISPLACEMENTS since then it will become redundant. See also this issue.