Removing ability to enable/disable parcel handling

[sithhell]

The main motivation behind this feature was to allow better interoperability
with MPI. IIRC, this feature didn't solve that problem.
By removing that, parcelport implementations can be simplified.

[hkaiser]

@eschnett You were the one asking for the feature in the first place. We're about to remove it - do you rely on it, still?

[sithhell]

From IRC:

[20:39:37] <heller> eschnett: i just want your oppinion on https://github.com/STEllAR-GROUP/hpx/pull/1778 real quick
[20:40:35] <eschnett> what does this remove, exactly?
[20:41:01] <heller> eschnett: https://github.com/STEllAR-GROUP/hpx/blob/38b57407619839365a5410d30705dd0e7b1eccc4/hpx/runtime/parcelset/enable_disable.hpp
[20:41:05] <heller> that thingy here
[20:41:34] <eschnett> when and where did i ask for this?
[20:44:09] <heller> eschnett: https://github.com/STEllAR-GROUP/hpx/issues/949
[20:44:10] <heller> here
[20:46:19] <eschnett> thanks. i don’t need this particular api; if i recall, it was proposed by hartmut.
[20:47:04] <eschnett> i need a way to ensure that hpx doesn’t make mpi calls at certain times.
[20:47:52] <eschnett> i’m sure there are other ways to achieve this, e.g. building hpx without mpi support
[20:50:04] <heller> eschnett: yes sure, that API was supposed to achieve this ... at quite a high cost, and never really worked out as intended ... 
[20:50:11] <heller> that's why I want to remove it ...
[20:50:21] <eschnett> ok
[20:50:43] <K-ballo> why didn't ever really worked?
[20:50:46] <eschnett> i can see how it’s difficult to achieve this without completely shutting down hpx, or taking a risk
[20:50:55] <heller> for example, it doesn't account for other ranks issuing send/receive requests to that locality