[TargetList] Delete the destructor

AFAICT, ~TargetList simply implements the default destructor, plus some
locking.

The history is murky, so I'm not sure why we do this locking. Perhaps,
at some point, it was possible to delete the same TargetList instance
from two different threads, setting up a race. If that were true, then
the locking would protect against the race.

Since TargetList is uniquely owned by Debugger (m_target_list), no such
race is possible today.

Testing: check-lldb

Differential Revision: https://reviews.llvm.org/D90895