Delete MAPL2 module and MAPL/ subdirectory

[tclune]

Summary

• Removes all #ifdef MAPL_SUPPORT_MAPL3 conditional blocks from MAPL/MAPL.F90 and MAPL/CMakeLists.txt
• Deletes the MAPL/ subdirectory entirely; the MAPL umbrella target is now solely provided by mapl3g/
• Removes add_subdirectory(MAPL) from the top-level CMakeLists.txt

Motivation

With all external clients migrated to use MAPL (PR2* series), the MAPL2 transitional module is no longer needed. This completes the MAPL2 retirement sequence.

Related Issues

Closes #4888

Dependencies

Requires all PR2* merges:

• Replace MAPL2 dependency with MAPL in GEOS_Shared GMAO_Shared#436 ✓
• Replace MAPL2 dependency with MAPL in CMakeLists GOCART#438 ✓
• Replace MAPL2 dependency with MAPL in GEOSgwd_GridComp GEOSgcm_GridComp#1416 ✓
• Replace MAPL2 with MAPL in source and CMakeLists FVdycoreCubed_GridComp#381 ✓
• Remove MAPL2 from CMakeLists dependencies GFDL_atmos_cubed_sphere#125 ✓