Revert "Improve immune spaces logic"

test is flaky. For example:

[ RUN      ] ImmuneSpacesTest.MultiImage
art/runtime/gc/collector/immune_spaces_test.cc:351: Failure
Value of: space5->Limit()
  Actual: 0xb6d72000
Expected: reinterpret_cast<uint8_t*>(spaces.GetLargestImmuneRegion().End())
Which is: 0xb6d93000
[  FAILED  ] ImmuneSpacesTest.MultiImage (1076 ms)

Bug: 27136196

This reverts commit 17b8bce0.

Change-Id: I9168421dd6ebabc271ed5c2cdbc5a27c211dcf5a