Remove unneeded LIBOSMOCORE_CFLAGS from tests
Remove unneeded LIBOSMOCORE_CFLAGS from tests

When testing for the presence of particular BTS model-specific header
during ./configure step, we don't need to add LIBOSMOCORE_CFLAGS because
none of those headers use it for compilation. Moreover, adding it might
hide the problem if the headers under check are available in the same
location where libosmocore headers were checked out.

Change-Id: I64bf1acb9db999567e8a2a6690cfc96d6e4b7ee1

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts