[m-rev.] for review: delete the hl grade component

[Zoltan Somogyi]

For review by Julien.

The change to NEWS is a bit lame. The reason is that the hl
grade component was already commented out in the user guide,
so it was not user visible, but the --high-level-data option *was*
user visible, so users could specify it anyway, if they really wanted.
If anyone knows a better wording for NEWS, I am all ears.

I also have a question. This diff deletes every occurrence of
the C macro MR_HIGHLEVEL_DATA, since it is now unnecessary,
but a bunch of occurrences of that string remain in C# code,
almost all in #ifdefs in library/rtti_implementation.m. The
#else parts of those #ifdefs seem to me to be relics of the
.NET backend, when it tried to access low-level-data RTTI.
Since compile_target_code.m *always* defined MR_HIGHLEVEL_DATA
for C#, I think the non-MR_HIGHLEVEL_DATA sides of those
#ifdefs are dead code, but I am not 100% sure. Does anyone
have any reason to believe that these apparently dead pieces
of code are in fact needed? And if not, is there any reason why
we need to continue defining MR_HIGHLEVEL_DATA for C#? 

Zoltan.
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