Simplify custom-builds tutorial

Remove, the disturbing goog.dom.ASSUME_STANDARDS_MODE define.
Remove jscomp_off stuff.

doc/tutorials/custom-builds.md

@@ -60,12 +60,8 @@ Creating a custom build requires writing a build configuration file. The format
       "externs/topojson.js"
     ],
     "define": [
-      "goog.dom.ASSUME_STANDARDS_MODE=true",
       "goog.DEBUG=false"
     ],
-    "jscomp_off": [
-      "unknownDefines"
-    ],
     "extra_annotation_name": [
       "api", "observable"
     ],
@@ -215,7 +211,6 @@ Now let's try a more complicated example: [`heatmaps-earthquakes`](http://openla
       "ol.ENABLE_WEBGL=false",
       "ol.ENABLE_PROJ4JS=false",
       "ol.ENABLE_IMAGE=false",
-      "goog.dom.ASSUME_STANDARDS_MODE=true",
       "goog.DEBUG=false"
     ],
     "compilation_level": "ADVANCED",