Remove picocoin reference configuration

Picocoin is no longer actively supported by the library, so the reference
configuration used in testing has been removed.
diff --git a/ChangeLog b/ChangeLog
index a6fa6bb..ab1ed29 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -113,6 +113,7 @@
    * Only run AES-192 self-test if AES-192 is available. Fixes #963.
    * Tighten the RSA PKCS#1 v1.5 signature verification code and remove the
      undeclared dependency of the RSA module on the ASN.1 module.
+   * Removed support for the library reference configuration for picocoin.
 
 = mbed TLS 2.6.0 branch released 2017-08-10