firmware/ice40-riscv/e1-tracer: Remove unused declaration of vio_set

This is a remanence of old firmware of the icepick on which that
design is based on.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: I0bc27da4faa91de983cd5f355553ecaa13754266
1 file changed