Remove ShouldDeferAsynchronousScriptsUntilAfterDocumentLoadOrFirstPaint preference

https://bugs.webkit.org/show_bug.cgi?id=319488
rdar://182295059

Reviewed by Tim Horton.

This has been stable for several years and there is no known SPI usage,
except a singular redundant internal call.

Canonical link: https://commits.webkit.org/317300@main