These flags weren't doing any real work, since the decision of whether some
source code is a script or module is made outside the parser (currently,
by the V8 API).

The only behavior change in this patch is to always parse 'import' and
'export' as their Token values, which changes the error message from
"Unexpected reserved word" to "Unexpected token import" (which doesn't
seem particularly harmful).

Review URL: https://codereview.chromium.org/1262913003

Cr-Commit-Position: refs/heads/master@{#30034}