Simplify block_lookup_symbol

One loop in block_lookup_symbol is identical to the code in
block_lookup_symbol_primary.  This patch simplifies the former by
having it call the latter.

This removes an assert.  However, note that the assert is not needed
-- it does not check any invariant that must be maintained.

Acked-By: Simon Marchi <simon.marchi@efficios.com>