encoding/json: remove unnecessary else conditions

encoding/json: remove unnecessary else conditions

Fixes golint warning about "if block ends with a return statement, so drop this else and outdent its block".

Change-Id: Id17ad0bf37ba939386b177b709e9e3c067d8ba21
Reviewed-on: https://go-review.googlesource.com/105736
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>