libprocessgroup: Remove PRODUCT_MEMCG_V2_FORCE_ENABLED

Commit f8901767 ("Add build flag to force memcg to the v2 cgroup
hierarchy") added the ability to ignore cgroups.json for the memory
controller, and force it to v2 when the PRODUCT_MEMCG_V2_FORCE_ENABLED
build flag is true. However the memory controller is now v2 by default
since commit 5fa48a84 ("libprocessgroup: memcg default version 1 -> 2")
so this build flag is no longer needed.

Bug: 327480673
Bug: 377579705
Change-Id: I46f45a44c01b2244eed2232ac091ef20c42cac41