remove setting of bowtie2_index to an empty channel

[emmanuel-tan]

Description

While working on nf-core/rnaseq#1731, noticed that ch_bowtie2_index is declared as a take: input to FASTQ_QC_TRIM_FILTER_SETSTRANDEDNESS, but the workflow body also re-initialized a local variable of the same name to channel.empty() before calling FASTQ_REMOVE_RRNA. This silently discards any pre-built bowtie2 index passed in by the caller, forcing FASTQ_REMOVE_RRNA to always build the index from scratch even when make_bowtie2_index = false and a real index path was supplied.

The re-initialization of ch_bowtie2_index to channel.empty() is removed to allow for a provided index to be used.

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