remove target from input of alignment steps

[anoronh4]

target is used as input for several alignment processes even though it is not used in the process. This could cause several steps to be re-run if target changes, even though they are not relevant to those steps. It might be helpful to remove target from channels for before SplitLanes or AlignReads steps, and then join the target back into the output channel after BQSR.

[gongyixiao]

This is a good point. The only process that uses target information is QcCollectHsMetrics

The ideal case is we don't need target column in fastq mapping.tsv at all. And the target info will be passed in only when QcCollectHsMetrics is enabled. But I'm not sure how we could achieve that without having another input tsv file, which will create another layer of complexity again. Not sure if the benefits worth the complexity.