Remove cluster metadata from SCE before merging

[sjspielman]

Does what it says! Since we aren't keeping clusters, we should not keep the cluster parameters.

This PR was brought to you by me actually writing docs 💪 and realizing it would be super weird to document this.

[sjspielman]

@allyhawkins before I merge this, I might actually want to make a few small other changes I'm noticing while writing docs. What do you think of doing these?

• We use the metadata field merged_hvgs but the individual object is highly_variable_genes. Should we change this to be merged_highly_variable_genes for consistency?
• Looks like we have named vector for library_id and sample_id. Should we unname?

  > metadata(sce)$library_id
  SCPCL000001   SCPCL000002   SCPCL000015 
  "SCPCL000001" "SCPCL000002" "SCPCL000015" 
  > metadata(sce)$sample_id
  SCPCL000001   SCPCL000002   SCPCL000015 
  "SCPCS000001" "SCPCS000002" "SCPCS000015" 

[allyhawkins]

We use the metadata field merged_hvgs but the individual object is highly_variable_genes. Should we change this to be merged_highly_variable_genes for consistency?

I don't have strong feelings about this one, but the longer one is more explicit so that's fine with me.

Looks like we have named vector for library_id and sample_id. Should we unname?

This stems from how we merge the metadata using the scpcaTools function for merging SCE's. I don't think it's a big deal to keep it and I also don't think specifying named vs. unnamed is important to mention in the docs. So if we change it later we don't have to worry about maintaining that in docs.

[sjspielman]

Can I get another 🐈 ? https://github.com/AlexsLemonade/scpca-nf/pull/657/commits/abb2a53ce613fdf622e5d05af1bb9b31fdc20949