Do not set request's window (#182)

Per whatwg/fetch#1823, this concept is going away. After that change, null-client scenarios will automatically do the right thing, and create no user prompts, which aligns with this spec's desired outcome.

Author: domenic

spec.bs

@@ -1289,8 +1289,6 @@ sequence=] |body|:
     ::  |body|
     :   [=request/client=]
     ::  `null`
-    :   [=request/window=]
-    ::  "`no-window`"
     :   [=request/service-workers mode=]
     ::  "`none`"
     :   [=request/initiator=]