remove `encryption key` section for certtool docs (#7820)
remove `encryption key` section for certtool docs (#7820)

This commit removes the encryption key section from
the certool.exe docs because:
 - MinIO does not support any TLS cipher that encrypts
   something with the private key. We only support PFS
   ciphers.
 - The doc comment is not really accurate anyway.
NitishT/minio,fwessels/minio,vadmeste/minio,donatello/minio,aead/minio,harshavardhana/minio,aead/minio,minio/minio,harshavardhana/minio,fwessels/minio,vadmeste/minio,harshavardhana/minio,harshavardhana/minio,harshavardhana/minio,aead/minio,aead/minio,donatello/minio,NitishT/minio,harshavardhana/minio,balamurugana/minio,harshavardhana/minio,aead/minio,balamurugana/minio,harshavardhana/minio,aead/minio,fwessels/minio,aead/minio,NitishT/minio,krisis/minio,krisis/minio,fwessels/minio,aead/minio,NitishT/minio,minio/minio