$locale was only used to set the LC_TIME locale.  Unfortunately, Mutt
previously defaulted to using "C".  This overrode the user's locale
setting and forced them to re-specify their locale inside their
.muttrc.

Remove $locale and instead use the locale specified by the
environment.  Mutt still allows "C locale" dates by using a leading
"!" in $date_format, ${}, etc.

Another use of $locale was to customize attribution dates using hooks.
The next commit will introduce $attribution_locale, which can be used
for this instead.

Thanks to Derek Martin for the original patch!