Revert "Make boolean expressions callable."

This reverts commit 59443fd7f917a2f7dcdd046360d2c89ceef514bd.

This is consistent with the behavior of the rest of Basic and Expr, which no
longer define __call__, but rather use .rcall().

Ping @rlamy.

Conflicts:
sympy/assumptions/tests/test_assumptions_2.py
sympy/logic/boolalg.py