Revert "[lldb] [Process] Watch for fork/vfork notifications" and associated followups

This commit has caused the following tests to be flaky:
TestThreadSpecificBpPlusCondition.py
TestExitDuringExpression.py

The exact cause is not known yet, but since both tests deal with
threads, my guess is it has something to do with the tracking of
creation of new threads (which the commit touches upon).

This reverts the following commits:
d01bff8c,
ba62ebc4,
e761b6b4,
a345419e.