Remove atllbuild option swiftc-path

This option was deprecated in atbuild 0.9.0.  Use `--toolchain` on the CLI instead.

Users should have had enough time to migrate at this point.
2 jobs for swiftc-path in 7 minutes and 48 seconds (queued for 5 minutes and 35 seconds)
Status Job ID Name Coverage
  Build
passed #4163
autoscale-linux
linux

00:06:23

passed #4164
openswift
osx

00:01:25