Bug 1650770 [wpt PR 24455] - Update interfaces/appmanifest.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/appmanifest.idl (#24455)

Source: https://github.com/tidoust/reffy-reports/blob/65872de/ed/idl/appmanifest.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/705383235
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wpt-commits: f302e398f01f26651869cdc8d799ff36824e4740
wpt-pr: 24455

testing/web-platform/tests/interfaces/appmanifest.idl

@@ -34,7 +34,6 @@ enum DisplayModeType {
 };
 
 dictionary ManifestImageResource : ImageResource {
-  USVString platform;
   USVString purpose;
 };