i2c: shell: Remove prefix filtering of device list

The I2C shell would filter the list of devices it knows about to
ones that have a device name that starts with "I2C_".  It was the
case that the majority of I2C bus controller devices happened
to be named with the "I2C_" prefix, however there is no guarantee
that is the case.  With the remove of label properties from the
devicetree this is even more true.

For now remove the prefix filter and just return the full list
of devices.

Signed-off-by: Kumar Gala <galak@kernel.org>