[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.
Merge pull request #1029 from ranma42/dont_free_global_codeman-cleanup

[runtime] Remove the last bits of mono_dont_free_global_codeman