Fixes a regression in forwarding from #6115 (#6191)

* Fixes a regression in forwarding from #6115

Although removing the authentication header is good defense in depth,
for forwarding mechanisms that use the raw request, we never add it
back. This caused perf standby tests to throw errors. Instead, once
we're past the point at which we would do any raw forwarding, but before
routing the request, remove the header.

To speed this up, a flag is set in the logical.Request to indicate where
the token is sourced from. That way we don't iterate through maps
unnecessarily.