remove PagedSeq from Scala 2.13

[SethTisue]

it was deprecated in 2.12.

recently it was copied into the scala-parser-combinators library, which was (hopefully) the only thing using it: scala/scala-parser-combinators#97

but there isn't yet a scala-parser-combinators release that has it, so we can't actually remove it from the standard library quite yet. well, I suppose we could speculatively remove it on the assumption that either a scala-parser-combinators will be published in time, or we'll unbundle in time. but anyway there's no rush, just something to get back to, hopefully for 2.13.0-M1.

[SethTisue]

(...if we removed it now, nothing would break unless an existing scala-parser-combinators version were republished for 2.13. but, we have actually done such republishings in the past...)

[SethTisue]

(at scala/scala-parser-combinators#99 I recently pinged Antoine about the possibility of rolling a scala-parser-combinators release)

[SethTisue]

note that the scala-parser-combinators repo has 1.0.x and master branches, the latter should probably be renamed to 1.1.x, and the latter is also where the PagedSeq removal happened. 1.0.x is being left alone, because bincompat

@gourlaysama recently published 1.0.6 but that doesn't help us here, we need a 1.1.x release

[SethTisue]

progress: scala/scala-parser-combinators#115

[SethTisue]

see scala/scala#6198