Revert "Wrap the user entrypoint function in a zone with native exception callback. (#7512)" (#7522)

This reverts commit 25559ed0.

Reason for revert: broken in AOT mode.

@pragma('vm:entry-point') placed on a function only instructs
the compiler to retain the function itself, but does not tell
compiler to generate and retain tear-off for this function.

In this PR _runMainZoned was marked as an entry-point but C++
code was trying to tear it off and use a closure, instead of
invoking it directly, which is not supported.