Remove unnecessary device_ checks and kDeviceNotOpened error message.
Remove unnecessary device_ checks and kDeviceNotOpened error message.

As of commit 47fc2064019a963683a0682237c0ac58f754f713 the Mojo Device
object tracks its own open state so WebUSBDeviceImpl will always have a
DevicePtr whether the device open or not. If the Mojo object has been
destroyed then the error handler set up by MakeScopedUSBCallbacks will
be run so checks in each method are not necessary.

BUG=None

Review URL: https://codereview.chromium.org/1377973002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#351428}

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