(#9793) "secure" indirector file backed terminus base class.

The file base class in the indirector trusted the request key directly, which
made it vulnerable to the same potential for injection attacks as other
terminus base classes.

However, this is somewhat mitigated by the fact that base class is entirely
unused. We can simple eliminate it from the system, because nothing is more
secure than code that doesn't exist.

The only consumer of the code was in the tests, and didn't care what base
class was used, so that was substituted with a continuing class.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>