CLEANUP: 51d: move global settings out of the global section

CLEANUP: 51d: move global settings out of the global section

We replaced global._51degrees with global_51degrees since there's no need
to store all this into the global section. This removes the last #ifdefs,
and now the code is 100% self-contained in 51d.c. The file 51d.h was now
removed because it was only used to load 51Degrees.h, which is more easily
loaded from 51d.c. It provides a good example of how to integrate code in
the future without touching the core parts.