Help: removed sort_sections key with invalid value

[ci skip]

At no point is "generated" checked for as value of that key (the only valid value is "yes").
"generated" is a valid value for sort_topics, however. As for whether we want sorting or not
for Eras, I don't think so. The era order as displayed seems logical enough since it groups
each era and Age of Heroes variant together.

data/core/help.cfg

@@ -61,7 +61,6 @@
         id=eras_section
         title= _ "Eras"
         sections_generator=eras
-        sort_sections=generated
         sort_topics=yes
         topics=..eras_section
     [/section]