mail: remove the SIGHUP handler from lex.c

If the command is simply going to exit without doing any cleanup, then
an immediate exit(3) does not add any value and in-fact obscures that
the program was terminated by a signal.

This is motivated by POSIX conformance requirements, but it's also a
little bit of a tidy-up. Some investigation into the history of mail(1)
shows that, at one point, hangup() would attempt to save messages into
mbox before exiting. It was later transformed into a simple exit() and
likely could have been GC'd at that point.

Reviewed by: des, emaste, ivy
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50247