mhlistsbr.c: Replace list_application() with body at call site.

mhlistsbr.c: Replace list_application() with body at call site.

list_application()'s comment said the function didn't need to exist.
It's correct, so delete it.  The sole caller now calls list_content(),
as list_application() used to do.  That's actually the same as other
cases in the switch so merge them.  There is a slight difference:
list_application() used to call list_content() and then return OK
regardless, now it returns list_content()'s value, but that's always OK
too.