Remove exceptions from try_lock in timed_mutex.
Remove exceptions from try_lock in timed_mutex.

A code-redundancy elimation attempt had resulted in exceptions
(potentially) being thrown from the try_lock, try_lock_for, and
try_lock_until functions in timed_mutex and recursive_timed_mutex. This
commit eliminates those exceptions to ensure standard-compliance.

meganz/mingw-std-threads