[perl #19135] string eval turns off readonlyness on lexicals

[perl #19135] string eval turns off readonlyness on lexicals

Don’t turn off readonliness on lexicals when freeing pad entries.

The readonliness is (prior to this commit) turned off explicitly in
pad_free under ithreads. See also bug #19022, which resulted from the
same change. There is some discussion there, too, but nobody seemed to
know exactly why the readonliness needed to be turned off.

Change 4761/2aa1bed, from January of 2000, added that SvREADONLY_off.
It is supposed to make sure that pad entries that were constants will
not be constants the next time they are used.

Dave Mitchell writes:
> I think...[this]...fix is correct (just removing the SvREADONLY_off).
> The issue it was trying to fix appears to have been properly fixed
> later by 3b1c21fabed159100271bd60bac3f870f5ac16af, which is why it's
> safe to remove it.

So this commit just deletes that code.