R1:04c07a7a7b6d

Remove the developer-specific CSRF help in phabricator_form()

Summary:
Fixes T4802. For context, see T1921.

Originally (in T1921), a developer ran into an issue where rendering `phabricator_form()` with an absolute URI confusingly dropped CSRF tokens, and it wasn't obvious why. This is a security measure, but at the time it wasn't very clear how all the pieces fit together. To make it more clear, we:

# expanded the exception text in developer mode to include a description of this issue; and
# added…