Relax deprecation messages

The deprecation message for the channel create and close
operation has been removed to not annoy users of pipeline
which want to stay with the current syntax.

However the deprecation remains for those methods when
using DSL2.

Related: #1269

modules/nextflow/src/main/groovy/nextflow/Channel.groovy

@@ -74,7 +74,7 @@ class Channel  {
     @Deprecated
     static DataflowChannel create() {
         if( NF.isDsl2() )
-            log.warn("The channel `create` method is deprecated -- it will be removed in a future release")
+            throw new DeprecationException("Channel `create` method is not supported any more")
         return CH.queue()
     }
 
@@ -128,6 +128,8 @@ class Channel  {
      */
     @Deprecated
     static DataflowVariable just( obj = null ) {
+        if( NF.dsl2 )
+            throw new DeprecationException("The operator `just` is not available anymore -- Use `value` instead.")
         log.warn "The operator `just` is deprecated -- Use `value` instead."
         value(obj)
     }

modules/nextflow/src/main/groovy/nextflow/extension/ChannelEx.groovy

@@ -92,7 +92,6 @@ class ChannelEx {
     static DataflowWriteChannel close(DataflowWriteChannel source) {
         if( NF.isDsl2() )
             throw new DeprecationException("Channel `close` method is not supported any more")
-        log.warn "The `close` operator is deprecated -- it will be removed in a future release"
         return CH.close0(source)
     }