Drop IsLimited in favor of IsReachable

Summary:
These two methods have had the same meaning, but inverted, since
110b62f06992d0fb989153afff2dc3aea62a674f. Having one name for a single
concept simplifies the code.

This is a backport of Core PR15138

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4538