Revert "build: Introduce CONCURRENCY_MAKE_FACTOR"

This reverts commit 42c0ad09df23b3494193d40e8cd6e0c1a4c13b61.

config/options requires 'bc' to calculate CONCURRENCY_MAKE_FACTOR
before scripts/checkdeps having any chance to tell the user
that 'bc' is a required dependency

for now, remove CONCURRENCY_MAKE_FACTOR. CONCURRENCY_MAKE_LEVEL is more
than enough