[dev.fuzz] testing: remove testing.RunFuzzTargets

[dev.fuzz] testing: remove testing.RunFuzzTargets

It is a legacy practice to expose these exported
testing functions, and is not needed for running
fuzz targets.

Change-Id: Ic300c9bfd15f4e71a1cea99f12c97d671a899f9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/262258
Trust: Katie Hockman <katie@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>