[CMakeLists] Remove unused SWIFT_INCLUDE_OUTPUT_INTDIR + comment out unnecessary logic

[practicalswift]

SWIFT_INCLUDE_OUTPUT_INTDIR appears not to be unused (let me know if I am missing some indirect usage path :-)):

$ git grep SWIFT_INCLUDE_OUTPUT_INTDIR
$

[modocache]

Thanks so much for this. Trimming down the build scripts is really helpful. Obsolete variables can lead people down rabbit holes--which happened to me just yesterday. 😃

[practicalswift]

@modocache Actually it was your PR #1410 that made me look into unused CMakeLists variables :-)

I wrote a small script that tries to find unused variables. I've found 18 candidates for deletion (including the one reported in #1410 and the two covered by this PR).

However, I'm a bit unsure about some of them so I'd love to have someone familiar with cmake/the general build process look in to each case to see if I'm missing some dependency/indirect usage. What is the best way to proceed? Should I submit a PR marked "WIP" and have it reviewed?

[gribozavr]

Just delete this code :)

[modocache]

Agreed. If there is indeed something we can do better with regards to installing gyb, we should document that in a ticket on https://bugs.swift.org (along with much more detail than present here).

[gribozavr]

This LGTM.

[practicalswift]

@gribozavr Updated and rebased as per your request :-)