This reverts commit b6f56da1, reversing
changes made to 32318db1.

Remove silly change on mbku/mbkv/mbkf at the surface: this is useless
and is not compatible with 1 layer configurations (e.g. SWG and
ICE_AGRIF). Revert changes to namelists in ICE_AGRIF

RK3 time stepping branch formerly source:/NEMO/branches/2021/dev_r14318_RK3_stage1 updated on git with main. SETTE partly tested : problem with AGRIF_DEMO.