Remove Go implementation of non-standard ECDHE_PSK_WITH_AES_128_GCM_SHA256.

The BoringSSL implementation was removed in
https://boringssl-review.googlesource.com/5051. No need to have the runner half
around.

Change-Id: I49857f4d01be161df89fb7df93a83240b6a8cb02
Reviewed-on: https://boringssl-review.googlesource.com/5751
Reviewed-by: Adam Langley <agl@google.com>