[codex] Remove MyMPILib from libneo

[krystophny]

Summary

Remove MyMPILib from libneo completely so libneo no longer owns MPI wrapper code, tests, docs, or top-level MPI build requirements.

What Changed

• delete extra/MyMPILib entirely
• remove the MyMPILib user doc from libneo
• remove the dedicated test_mympilib coverage from libneo
• drop MyMPILib usage from test_arnoldi
• remove the top-level MPI requirement and add_subdirectory(extra/MyMPILib)
• keep the remaining Python-driven tests runnable by setting the required PYTHONPATH in CTest

Why

MyMPILib is not libneo core functionality. Keeping it inside libneo forced MPI into the default build surface and blurred ownership with NEO-2.

Impact

• libneo no longer exports or builds MyMPILib
• libneo no longer requires MPI at top level for its normal build
• NEO-2 now owns the moved code in the companion PR

Verification

cmake --build --preset default
ctest --preset default

Result:

100% tests passed, 0 tests failed out of 71

Related

• Companion NEO-2 PR: [codex] Move MyMPILib into NEO-2 NEO-2#66
• Related issue: Dependency audit: downstream map, duplication report, and split proposals #258