pp_initfield: no need to extend av, should be the desired size

This was probably an accidental leftover after an early pull
request changed AV creation to `av = newAV_alloc_x(count)`
just above the removed line.

class.c

@@ -78,7 +78,6 @@ PP(pp_initfield)
 
                 av = newAV_alloc_x(count);
 
-                av_extend(av, count);
                 while(svp <= PL_stack_sp) {
                     av_push_simple(av, newSVsv(*svp));
                     svp++;