encoding/json: remove legacy fuzz.go file

encoding/json: remove legacy fuzz.go file

With native support for fuzzing in the Go toolchain,
rely instead on the fuzz tests declared in fuzz_test.go.

Change-Id: I601842cd0bc7e64ea3bfdafbbbc3534df11acf59
Reviewed-on: https://go-review.googlesource.com/c/go/+/471197
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>