Fix undefined reference advancex_

[Sichao25]

Building M3DC1 with branch #97 in Stellar encountered the error with intel compiler:

Casefiles/m3dc1_library.dir/particle.f90.o: In function 'particles_mp_particle_pressure_rhs_': /home/ur8212/sourceCodes/M3DC1/M3DC1/unstructured/particle.f90:3340: undefined reference to 'advancex_'

Since vspdims is currently hardcoded, it should be safe to comment out this code for now.

There are also some other references to vspdims.eq.5; some are already commented out, while others are not. Please let me know whether we will never need vspdims == 5, in which case we can remove all of these references, or whether vspdims is expected to be switched manually and the code recompiled when needed.

[jacobmerson]

@nferraro can you comment on if you are happy with this change. This is one of the final blockers for the updated build system.

[nferraro]

It's fine with me. I don't see advancex defined anywhere so this is probably some vestige from a different branch or debug test. In the former case, that can get resolved whenever it's merged.