Modules already have a separate entrypoint into the engine (at the moment,
this is v8::ScriptCompiler::CompileModule, though that will change to
something like ParseModule). This meant that requiring a commandline flag
simply added an extra complexity burden on embedders. By removing the v8
flag, this lets embedders use their own flagging mechanism (such as d8's
"--module", or Blink's RuntimeEnabledFeatures) to control whether
modules are to be used.

Also remove old modules tests that were being skipped (since they test
very old, pre-ES2015 modules syntax).

R=littledan@chromium.org
BUG=v8:1569, chromium:594639
LOG=y

Review URL: https://codereview.chromium.org/1804693002

Cr-Commit-Position: refs/heads/master@{#34764}