Remove mbed-os library

This commit reverts the addition of an mbed-os.lib file introduced with the Mbed-OS port. Mbed-OS build tools use .lib files to pull in additional dependencies automatically with a single command. The intended usage of mcuboot with mbed-os is to clone mcuboot *next to* mbed-os, rather than mbed-os inside mcuboot.

Signed-off-by: George Beckstein <becksteing@embeddedplanet.com>