[dart2wasm] Remove 100k string limit workaround

The issue of 100k limit on Firefox/Safari was discovered by us and we
filed bugs for firefox & webkit ([0] and [1]). After wasm spec
discussions in [2] it was determined that this limit isn't specified and
the implementations should remove this limit.

As this was a implementation bug and implementations have fixed it,
we're removing the workaround in this CL. That does mean that older,
buggy browser versions will have issues running dart2wasm code that uses
very large strings, but believe in pratice this is unlikely to hit
anyone (most users are on evergreen browser versions, apps unlikely
embed 100k large string constants). If an app suffers from this issue it
can use it's runtime detection logic to fallback to dart2js on the buggy
browser versions.

Doing this will allow very large internal app to move very large strings
to deferred wasm module instead of the main wasm module (which turns
150 MB mjs file into 100 kb mjs file)

[0] https://bugs.webkit.org/show_bug.cgi?id=299393
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1990202
[2] https://github.com/WebAssembly/design/issues/1574

Change-Id: I2a1aa5fe2b9600cec0d6869102060cdd62ff7c18
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/464741
Reviewed-by: Nate Biggs <natebiggs@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Kevin Moore <kevmoo@google.com>