Don't generate plugin registry in ResidentWebRunner (#91281)

* Don't generate plugin registry in ResidentWebRunner

generateDartPluginRegistry was being set to true unconditionally in
ResidentRunner, bypassing the primary check in
DartPluginRegistrantTarget, and the targetPlatform was not set in that
codepath, bypassing the second after the changes in
https://github.com/flutter/flutter/pull/87991. This caused web hot
restarts to be slower due to doing unnecessary work.

This ensures that generateDartPluginRegistry is false in the
ResidentWebRunner to skip that unnecessary step.

Fixes https://github.com/flutter/flutter/issues/91262

* Formatting

Co-authored-by: Zachary Anderson <zanderso@users.noreply.github.com>

Co-authored-by: Zachary Anderson <zanderso@users.noreply.github.com>