[runtime] Remove the last bits of mono_dont_free_global_codeman

All uses of the mono_dont_free_global_codeman variable (including its
definition) have been removed in
4a1ae4694a2d4fc06bd1cb02b88dfeeeb8c90630.

Its declaration should be removed as well to keep the codebase clean.
[mono-api-html] Add the [Obsolete] message for API additions and removals

Less common case - unless the API difference has a large delta.

Some additional spacing is added to ease readability: if an entry takes two line then it needs a empty line.
Merge pull request #1029 from ranma42/dont_free_global_codeman-cleanup

[runtime] Remove the last bits of mono_dont_free_global_codeman
use mkdir -f to ignore "directory already exists" error.
fix previous change (huh, I was blind, no such option -f, use -p).