Delete Revit specific icons from Dynamo repository

#### Purpose

Remove from Dynamo repository Revit specific icons. They were added to DynamoRevit repository in https://github.com/Benglin/DynamoRevit/pull/1.

#### Additional references

Delete part of task [MAGN-5672](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5672) DUI: Move Revit specific icons to DynamoRevit project.

Should be merged before https://github.com/Benglin/DynamoRevit/pull/1.

#### Reviewers

@Benglin, please, take a look.