Remove target IsNavigationAllowed() check in FrameLoader::StartNavigation.

This will be checked again when on the target frame's StartNavigation.

Change-Id: I013e34edab3ca010091e1cd0e98e9c1758c8c3f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534649
Auto-Submit: Nate Chapin <japhet@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643966}