Remove without-interrupts from runtime/intrpt.scm.

Remove without-interrupts from runtime/intrpt.scm.

It was only used in the internal install procedure where an "atomic"
updated was described.  Punted that and assumed the procedure is not
run in multiple threads concurrently.  It should be called only during
the single-threaded cold load or in a careful developer's REPL(?).

Remove set-fixed-objects-vector! too; assume the fixed objects vector
is not a copy.