Remove duplicate invalid-superclass handling in SHARED-INITIALIZE :AFTER STD-CLASS
* In
fe027d69, a new condition and a generic function, both named
INVALID-SUPERCLASS, have been introduced. As
expected, (SHARED-INITIALIZE :AFTER STD-CLASS) delegates signaling the
invalid-superclass problem to INVALID-SUPERCLASS.
* However, in
fe027d69, the previous code for "directly" signaling a
SIMPLE-ERROR remained in (SHARED-INITIALIZE :AFTER STD-CLASS),
probably by mistake.
* This commit removes the redundant code and slightly tweaks the report
of the INVALID-SUPERCLASS condition.