FlatAffineConstraints::removeId() fix.

This is an obvious bug, but none of the test cases exposed it since numIds was
correctly updated, and the dimensional identifiers were always eliminated
before the symbolic identifiers in all cases that removeId was getting
called from. However, other work in progress exercises the other scenarios and
exposes this bug.

Add an hasConsistentState() private method to move common assertion checks, and call it
from several base methods. Make hasInvalidConstraint() a private method as
well (from a file static one).

PiperOrigin-RevId: 224032721