icecc: remove bb.errors on un-built compilers

icecc: remove bb.errors on un-built compilers

Building things with icecc and BB_NUMBER_THREADS more then 1 you could see
a lot of messages:

ERROR: no cross compiler built yet?

before gcc-cross would be properly built. There is technically nothing wrong,
cross compiler is not there yet and it's not needed when bitbake does
fetch-unpack-patch and other things to prepare package build. But it makes
people uncomfortable.

We can safely kill this messages.