Remove unused stats from cache structure.

Removing the stats make the whole cache structure fit in a single page.

Bug: 131362671

Test: Verified that all bionic malloc benchmarks are still the same.
Test: It turns out that the malloc_sql benchmarks seem to get faster.
Test: Verified that after this change, it saves about 2K PSS per thread.
Change-Id: I4dcd633543f05f1a9d47db175f9977ddb42188a9
(cherry picked from commit a8b52518654725e64def5ebbd0ed472100e5a522)