Remove rules targetting non-existent posterior

[ascholerChemeketa]

Fixes hint placement identified by Chrissy in this sample:

Example 1.1.7: Sony and The Interview https://tanaquil18.github.io/matrixtheory/sec-lin-eqn-substitution.html#subsec-lin-eqn-dc-13

[rbeezer]

Thanks very much! Fairly isolated situation - I had to make an example and turn on knowlization for its hints, etc. But testing confirms the fix.

git log -S posterior is interesting. Second commit is yours, and oldest is mine. ;-)

Website update soon, but not just now.