log: remove dead code to follow descendants if ascending revisions specified
It's been disabled since 8b4b9ee6001a "log: make -fr show complete history
from the given revs" 2015-02-06, and we've got no bug report regarding this
exact behavior. For details what the heck "followdescendants" is, please see
the issue comment:
"'rev(%d)' % startrev" can be replaced with '.' since revs are 'reversed(:.)'.