Remove phase of moon feature

The computation never really worked before and it seems like the feature
is not very helpful, sometimes even confusing (see GitHub issue #21).

The macro ISLEAP is moved to calcurse.h.

Signed-off-by: Lars Henriksen <LarsHenriksen@get2net.dk>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/ui-day.c

@@ -916,7 +916,6 @@ static char *fmt_day_heading(time_t date)
 
 	localtime_r(&date, &tm);
 	string_init(&s);
-	string_printf(&s, "%s ", ui_calendar_get_pom(date));
 	string_catftime(&s, conf.day_heading, &tm);
 	return string_buf(&s);
 }