Remove unwarranted assertion in Perl_newATTRSUB_x()

Remove unwarranted assertion in Perl_newATTRSUB_x()

RT #126845: if a stub subroutine definition with a prototype has been seen,
then any subsequent stub (or definition) of the same subroutine with an
attribute was causing an assertion failure because of a null pointer.

This assertion was added in 2eaf799e74b14dc77b90d5484a3fd4ceac12b46a, which
itself would already have triggered this assertion failure, even though all
subsequent uses of the pointer in question were guarded with non-null
conditions. So merely deleting the assertion is the right thing.