timedate: Drop custom logging macros in favor of log context

timedate: Drop custom logging macros in favor of log context

Additionally, using the log context makes sure the extra fields are
applied to all log messages generated while the context is in place,
rather than only log messages logged with log_unit_xxx() in timedated
itself.

It also means the unit name is prefixed to all log messages logged
within that context. While it's not clear whether we always want the
unit name to be attached to library log messages, we also don't want
the unit name to not be attached to any library log messages, so we opt
to add more rather than less information by adding the unit name everywhere
for now.

In the future we can look into some log.h helpers to enable/disable
adding the prefix to the following log messages or not.