Remove handle as a keyword

Owned/unique handles to resources are now just directly named, and
this keyword is unused.

Author: kulakowski-wasm

design/mvp/WIT.md

@@ -94,7 +94,6 @@ keyword ::= 'use'
           | 's8' | 's16' | 's32' | 's64'
           | 'float32' | 'float64'
           | 'char'
-          | 'handle'
           | 'record'
           | 'enum'
           | 'flags'