gc.c: speed up RGENGC_CHECK_MODE by removing unncessary check

* gc.c (reflist_add): revert changes from r44109. it is unnecessary
  after r44113
* gc.c (allrefs_i): fix whitespace
* gc.c (allrefs_roots_i): fix whitespace

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44114 b2dd03c8-39d4-4d8f-98ff-823fe69b080e