Remove PRODUCT_MEMCG_V2_FORCE_ENABLED

Commit 18a5d392 ("Add flags for cgroup v2 and memcg v2 features")
added a build flag that allows forcing the memory cgroup controller to
v2 when PRODUCT_MEMCG_V2_FORCE_ENABLED is true. However the memory
controller is now v2 by default since commit 5fa48a84b
("libprocessgroup: memcg default version 1 -> 2") so this build flag is
no longer needed.

Bug: 327480673
Bug: 377579705
Change-Id: I770efa71d8f57abee58adc2be4bac097d35e3b34