Drop the support of synchronous execution

[huningxin]

Remove the definition and algorithm steps for

• ML.createContextSync()
• MLGraphBuilder.buildSync()
• MLContext.computeSync()

This PR also removes the synchronous execution description from Programming Model section.

The previous algorithm steps of MLGraphBuilder.build() depends on MLGraphBuilder.buildSync(). The algorithm steps of MLGraphBuilder.buildSync() have issue tracked by #457. That is left to a separate PR. This PR just merges the algorithm steps of MLGraphBuilder.buildSync() into MLGraphBuilder.build() with some modifications:

• Remove "The permissions and context validity have been checked..." note regarding to #501.
• Change throwing exception to rejecting the promise with exception.

Fix #531

@wchao1115 @anssiko @inexorabletash , PTAL. Thanks!

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Preview | Diff

[inexorabletash]

The bots seem to be having issues, but I've locally verified that there are no Bikeshed warnings or errors.

[zolkis]

PR preview failed, just wanted to call attention to common usage of "execute graph" and other algorithms from the "Synchronous execution" section also in asynchronous execution. These should be moved or merged into the async execution steps. It's hard to see from the diff if that was the case.

[anssiko]

By merging, we're checking some boxes:

• [x] This removal gets into the next Candidate Rec Snapshot (CRS) as anticipated in the high-level status https://www.w3.org/TR/webnn/#sotd
• [x] This also addresses feedback from earlier TAG review https://github.com/w3ctag/design-reviews/issues/771#issuecomment-1332346304 that suggested to drop this feature. (I gave TAG an update on this removal in our delta review.)