nmxact: Remove "Serialized OMP" debug log message

This message got logged immediately before the request got transmitted.
The result was two nearly identical large log messages (both contain the
full payload of the request).

In addition, the image upload command actually serializes several
requests for each one it sends.  It does this to determine the ideal
chunk size to use for the next request.  The result is even more noise
in the logs
diff --git a/nmxact/omp/omp.go b/nmxact/omp/omp.go
index 8691c10..e36a8f1 100644
--- a/nmxact/omp/omp.go
+++ b/nmxact/omp/omp.go
@@ -20,12 +20,10 @@
 package omp
 
 import (
-	"encoding/hex"
 	"fmt"
 
 	"github.com/fatih/structs"
 	"github.com/runtimeco/go-coap"
-	log "github.com/sirupsen/logrus"
 	"github.com/ugorji/go/codec"
 
 	"mynewt.apache.org/newtmgr/nmxact/nmcoap"
@@ -157,11 +155,6 @@
 		return nil, fmt.Errorf("Failed to encode: %s\n", err.Error())
 	}
 
-	log.Debugf("Serialized OMP TCP request:\n"+
-		"Hdr %+v:\n%s\nPayload:%s\nData:\n%s",
-		nmr.Hdr, hex.Dump(er.hdrBytes), hex.Dump(er.m.Payload()),
-		hex.Dump(data))
-
 	return data, nil
 }
 
@@ -176,10 +169,5 @@
 		return nil, fmt.Errorf("Failed to encode: %s\n", err.Error())
 	}
 
-	log.Debugf("Serialized OMP datagram request:\n"+
-		"Hdr %+v:\n%s\nPayload:%s\nData:\n%s",
-		nmr.Hdr, hex.Dump(er.hdrBytes), hex.Dump(er.m.Payload()),
-		hex.Dump(data))
-
 	return data, nil
 }