Remove _strlen31

Remove _strlen31

This function is old and fairly broken.  Code archeology in our git tree
hasn't revealed why it was creted (though it may have possibly been to
support older win32 systems that couldn't do 64 bit integers properly,
like windows 95/98).

There seems to be no good reason to keep it around, and given that it
has potentially serious side effects, lets just remove it.

Fixes #27761

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27763)