[zircon][usb-request-cleanup]Remove bti_handle from usb-request

The bti_handle is used only during the physmap which is used only at the
usb infrastructure level and not by the client drivers. This change removes
the bti handle from usb_request_t, and also removes the need for it beeing needed
during the usb_request_alloc. In a later changeset, clients will be using
the usb_request_alloc directly from the usb-request library and not via
the protocol op. This changeset is in preparation for that to work.

Test: Full build. Netbooting on vim2. Plugged in a usb-mass-storage
device and verified lsusb works as expected.

Change-Id: I158917a9cfec1c3b0f2b9ec905c17dbfcc6ecbc7

[ddk][test] Clean up ddk tests

Moved usb-request test to system/dev/test/ddk-test.
We can use this as a central location for adding new tests for DDK APIs.

Removed obsolete iotxn-test.

TEST: runtests in qemu, verified that driver-tests run usb-request and ddktl tests successfully

Change-Id: I63bbef51660c0af037958888567e13ae10afc9fc