scheduler: Remove assert on the exact number of weighers

Since the weighers code is by design pluggable, downstream packagers may have
added additional weigher classes, causing this assert to fail.

Rather than require downstream maintain a patch against this line, the
proposal is to remove it since the asserts after it are sufficient.

Change-Id: I13765fae188af92a2e729dcd14caf981db01acb4
Merge "scheduler: Remove assert on the exact number of weighers"