[dev.fuzz] testing: cleanup a few small things
[dev.fuzz] testing: cleanup a few small things

Deletes the exported testing.Fuzz function
which would run a standalone fuzz target.
Similar to RunFuzzing and RunFuzzTargets,
which were previously removed, this will
likely be too complex to support.

Moves the deferred Exit in f.Fuzz higher up
the function so it is always run.

Change-Id: I9ea6210dc30dee8c2a943bfb8077225c369cfb95
Reviewed-on: https://go-review.googlesource.com/c/go/+/263642
Trust: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Run-TryBot: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,golang/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go