Revert "encoding/json: implement Is on SyntaxError"

This reverts CL 253037.

Reason for revert: The recommended way to check for a type of error is errors.As. API changes should also start with a proposal.

Change-Id: I62896717aa47ed491c2c4775d2b05d80e5e9cde3
Reviewed-on: https://go-review.googlesource.com/c/go/+/254837


Trust: Damien Neil <dneil@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>