Remove references to defunct swift content

After moving the swift system requirements and example
architecture to the overview chapter, I think we can
safely remove references to the defunct sections in the
swift chapter. A future patch will remove the defunct
files. Also improved some phrasing/wording.

Change-Id: I364f6d21dafbed22990bafb53c828b8e54775f85
Partial-Bug: #1389382
backport: juno
(cherry picked from commit dcfb255c55edbd9c43eb5b7db7a4eea4fc980c96)
Merge "Remove references to defunct swift content" into stable/juno