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Remove MIDI support from the core's main TinyUSB.  MIDI is still supported
using the Adafruit TinyUSB library, just like before (the core never
did use it).

Compile the Pico-SDK using `-Os`

Remove unneeded warning in UF2 upload

Blink.ino shows a savings of 3.5KB flash and 400 bytes of RAM.

For comparison, with this PR blink.ino reports:
````
Sketch uses 49908 bytes (3%) of program storage space. Maximum is 1568768 bytes.
Global variables use 7024 bytes (2%) of dynamic memory, leaving 255120 bytes for local variables. Maximum is 262144 bytes.
````

As comparison, using the MBED core blink.ino reports:
````
Sketch uses 78882 bytes (0%) of program storage space. Maximum is 16777216 bytes.
Global variables use 42780 bytes (15%) of dynamic memory, leaving 227556 bytes for local variables. Maximum is 270336 bytes.
````

So, with this PR we use **37% less flash** and and **87% less RAM**
for simple sketches.