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.
restore clisp buildability

fixes lp#1331141; thanks to Vasily Postnikov
really fix clisp build

Thanks again to Vasily Postnicov
Avoid calling SEQUENCEP in the XEP for some seq-dispatching functions.