array.c: fix false assertions

* array.c (rb_ary_sort_bang): remove duplicated assertions.
  ARY_HEAP_PTR() implies ary not to be embedded.  [ruby-dev:47419]
  [Bug #8518]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41253 b2dd03c8-39d4-4d8f-98ff-823fe69b080e