Remove emacs-pretest-bug address from documentation

[Glenn Morris]

I suggest that it's time to remove the emacs-pretest-bug address from
all documentation (except as a historical footnote), and from M-x
report-emacs-bug. It hasn't existed as an independent list for some
time, and has been just an alias for bug-gnu-emacs for well over a
year:

http://lists.gnu.org/archive/html/emacs-devel/2008-06/msg00790.html

The current system seems to be working fine, and it seems unlikely
e-p-b will ever be a separate list again. Having it still mentioned in
the documentation has caused some confusion when people find what
appears to be a dead list.

I will make the necessary changes if this is considered a good idea.