Retire Illumina merged platforms and switch directly to generic gene platforms instead

[arteymix]

We use merged platform because adding RNA-Seq data require that the same platform is used for all the samples. This is unnecessary because ultimately these platforms get replaced by a generic expression platform.

• human_illumina_rnaseq
• mouse_illumina_rnaseq
• rat_illumina_rnaseq

It also creates a burden on curators who have to switch to these generic sequencing platform before switching to a generic gene platform.

Given that this step is superfluous, we don't need to check for multi-platform when adding RNA-Seq data, and we should switch directly to the gene platforms.

[ppavlidis]

I agree that this makes more sense than my proposal in #1105, as long as it's always the correct thing to do.

[arteymix]

We're switching directly to the generic platform for single-cell experiments. I think we should reuse that logic for bulk RNA-Seq.