Targets no longer in the pipeline are recycled anyway

[wlandau]

Prework

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Description

If I remove a target from tar_pipeline(), tar_make() still tries to use it.

Reproducible example

library(targets)
tar_script({
  options(crayon.enabled = FALSE)
  tar_pipeline(
    tar_target(a, 1),
    tar_target(b, a)
  )
})

tar_make()
#> ● run target a
#> ● run target b

tar_script({
  options(crayon.enabled = FALSE)
  tar_pipeline(
    tar_target(b, a)
  )
})

tar_make()
#> ✓ skip target b
#> ✓ Already up to date.

^{Created on 2020-10-15 by the reprex package (v0.3.0)}

Expected result

tar_make() should have thrown an error because target a is no longer in the pipeline.