Merge #17469: test: Remove fragile assert_memory_usage_stable

Summary:
fac942ca57dce6cfa5655a3ac8664d6a051bc01f test: Remove fragile assert_memory_usage_stable (MarcoFalke)

Pull request description:

This test fails on arm64 and a fuzz tests seems inappropriate for the functional test suite anyway, so remove it.

Example failures:

* https://travis-ci.org/bitcoin/bitcoin/jobs/611497963#L14517
* https://travis-ci.org/MarcoFalke/bitcoin-core/jobs/611029104#L3876

ACKs for top commit:

jamesob:
  ACK https://github.com/bitcoin/bitcoin/pull/17469/commits/fac942ca57dce6cfa5655a3ac8664d6a051bc01f

Tree-SHA512: 3577e7ce5891d221cb798454589ba796ed0c06621a26351bb919c23bc6bb46aafcd0b11cb02bbfde64b74d67cb2950da44959a7ecdc436491a34e8b045c1ccf4

Backport of Core PR17469

I've skipped backporting Core PR14794 which attempts to fix the issue using a not-so-robust approach of
looking at env variables to determine if the memory usage threshold should be modified.

This fixes sanitizer failures on CI by removing the problem entirely.

Test Plan:
ninja check-functional
and run ASAN on CI

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Subscribers: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6202