Revert "prereq-build: check for "bc" (#13767)"

Makes all buildbot builds fail; until we get the right software
installed, revert this changeset.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37122 3c298f89-4303-0410-b956-a3cf2f4a3e73