r787: use mem_seed_sw() also for non-PacBio reads

In the previous version, mem_seed_sw() is only used for PacBio reads to filter
bad seeds. For non-PacBio long queries, bwa-mem uses mem_chain2aln_short() for
a similar purpose. However, it turns out that mem_chain2aln_short() is not
effective given long near-tandem repeats. Bwa-mem still wastes a lot of time
of futile ref substring and extensions.

In this commit, mem_chain2aln_short() has been removed. mem_seed_sw() is used
if the query sequence is long enough (~700bp). For shorter reads, the results
should be almost identical to the previous version.