Do not check tcache clear when doing a purge.

On systems where the tcache is disabled, the tcache clear option will
fail, and not really do a purge.

Bug: 137037462
Bug: 136236080

Test: Built taimen as malloc svelte and verified the M_PURGE mallopt
Test: call failed before this fix and passes afterwards.
Change-Id: Ib30e5f3e083a9c6d046adff30f2aa7eacaf6df10
Merged-In: Ib30e5f3e083a9c6d046adff30f2aa7eacaf6df10
(cherry picked from commit 3d0bafb945bc9e39ddc84a0167e713b8e70dc135)