base: Remove non-caching discardable memory implementation.
This removes the "malloc" implementation of discardable memory.
This implementation exists for test and debugging purproses but
we currently don't have any tests that depend on it so it can be
removed to reduce code size.
This implementation can still be very useful to manually track
down bugs and performance issues in discardable memory clients
but can be maintained separately to reduce code size.
BUG=422953
TBR=danakj@chromium.org
Review URL: https://codereview.chromium.org/
651913003
Cr-Commit-Position: refs/heads/master@{#299374}