[C++] Remove GlobalForkSafeMutex

[asfimport]

Now that we have a proper at-fork facility, the GlobalForkSafeMutex has probably become pointless and therefore can be removed.

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] Create after-fork facility (requires)

_{Note: This issue was originally created as ARROW-18271. Please see the migration documentation for further details.}