Remove unused Fortran imports

[krystophny]

Removes unused Fortran imports exposed by the full-project lint gate.

• Keeps MPIRoot conditional on MPI where SWRITE expands to a root-only write.
• Leaves the serial source inventory free of unused-import findings.
• Builds both MPI and serial configurations.

Verification

Test fails on develop

$ fo
Static: OK (86 modules, 86 changed, 86 affected)
Build: OK
Tests: skipped, no affected tests
Lint: FAIL (20 unused imports)
STOP 1

Test passes after fix

$ FO_CMAKE_ARGS='-DUSE_MPI=ON' fo build

$ FO_CMAKE_ARGS='-DUSE_MPI=OFF' fo build

Both configuration builds exited 0.

$ fo
Static: OK (98 modules, 98 changed, 98 affected)
Build: OK
Tests: skipped, no affected tests
Lint: OK
All stages passed (.9s)