Remove text on anonymous enums in ABI chapter

[schulzm]

Remove the following plus its ratioal

0.3.4 Integer Constants
Integer constants fall into groups, where all constants in each group must have unique values. To enforce unique values, groups of integer constants are defined in anonymous enums.
Rationale. Using anonymous enums, rather than preprocessor macros, may provide more useful debugging information in the application binary. (End of rationale.)

[abouteiller]

Question: using enums is not needed anymore because the ABI enforces uniqueness anyway. But having enums may still help with debuggers showing appropriate symbols. So the rationale is not justified anymore, but it could become an advise to implementors rather than being removed (to keep the part about playing nice with debuggers).

@devreal noted that using enums can cause problems with C++, which may be another reason to remove the rationale/advise altogether.

Note that the subsection is concerned only with integer constant (e.g., MPI_IDENT), not with handles (e.g., MPI_COMM_WORLD, which are in the prior subsection, and cannot be tracked back to their constant value in debugger due to the required casting in their definition).

There are not that many of such enumerated constants (attribute keyvals, group comparison results, thread level, datatype combiners, split-type constants, and a bunch of MPI_T scope, bind, pvar attributes).

[devreal]

I should clarify that: using enums works with C++ if they are not scoped (i.e., regular enum {}; works but not enum class {};).

[devreal]

I agree that an AtoI would be a good idea. If not in this round, it could be added later since that won't change the ABI.

[jeffhammond]

the stubs header uses enum but an implementation can use #define or another mechanism if it wants, as long as the integer constant values are correct.
https://github.com/mpi-forum/mpi-abi-stubs/blob/main/mpi.h