Bug 1779138 - Disable GatherTotalMemory on MacOS. r=florian, a=RyanVM

In order to remove uses of ResidentUnique on MacOS this patch disables
GatherTotalMemory in its entirety (rather than provide inaccurate telemetry).
I will follow up with a patch that uses task_info phys_footprint to re-enable
it.

Differential Revision: https://phabricator.services.mozilla.com/D156048