ice40/pack: During IO packing, remove any unused input connection
ice40/pack: During IO packing, remove any unused input connection

This is mostly for the benefit of PLL placement because the D_IN_x
ports are used for other purposes when PLL is enabled so we need to
make sure nothing is connected there already. (even an unused net is
too much)

Signed-off-by: Sylvain Munaut <349f318521b45672c77207abf1a5ed60eb011db1@246tNt.com>

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