[dev.typeparams] internal/goexperiment: regenerate generated files

Rerun the generator. exp_regabi_{on,off}.go are gone, as "regabi"
itself is not a goexperiment that we test at run time (the
sub-experiments are).

Change-Id: Ic1f31b4ef2769a143f768e1b3dc7221041aafca9
Reviewed-on: https://go-review.googlesource.com/c/go/+/325912


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>