Fix/run slsi.std assay no npcs binding

[dcollins15]

Drops reference to npcs variable from RunSLSI.StdAssay. Since npcs is not defined, the following NOTE to be thrown during CRAN checks:

❯ checking R code for possible problems ... [40s/40s] NOTE
  RunSLSI.StdAssay: no visible binding for global variable ‘npcs’
  Undefined global functions or variables:
    npcs

In lieu of any tests, the following script can be used to quickly verify RunSLSI is still working as expected:

library(Seurat)
library(Signac)
library(BPCells)

# Convert "peaks" to an `Assay5` instance.
atac_small[["peaks"]] <- as(atac_small[["peaks"]], Class = "Assay5")
# `RunSLSI` should not raise any errors when passed a `StdAssay`.
atac_small <- RunSLSI(
  atac_small,
  graph = "peaks_snn",
  assay = "peaks"
)

# The same should be true for an `Assay5` instance holding a
# `BPCells::IterableMatrix`.
atac_small[["peaks"]]$data <- as(
  atac_small[["peaks"]]$data,
  Class = "IterableMatrix"
)
atac_small <- RunSLSI(
  atac_small,
  graph = "peaks_snn",
  assay = "peaks"
)

Relates to:

• Revamp CI checks #9538