Fix the bogus MFEXPR* property for MPOIS

Fix the bogus MFEXPR* property for MPOIS

The MFEXPR* prop for MPOIS has been the list (LAMBDA (X) X) instead
of the desired identity function.  Attempting to apply this list to
args would cause a lisp error.

Here's one way to trigger the bug:

(%i1) define (foo (), intopois (1))$

(%i2) foo ();
<lisp error>

This used to work in Macsyma under Maclisp and some other older
lisp dialects.

This bogus definition for the MFEXPR* prop for MPOIS is in pois3.
It turns out that a valid definition for this is in pois2.  Given
the way that Maxima is being built, the definition from pois2 is
being replaced by the one in pois3.

I'm removing the definition from pois3, and moving the definition
from pois2 to pois3.  The move is just because there is already a
comment in pois3 about this property.

No problems with the test suite or share test suite.