Remove use of deprecated -P:scalanative:mapSourceURI flag

[mpilquist]

Currently blocking scalacheck from building for 0.5 (without adding custom build configuration to filter this flag out), which in turn is blocking munit and so on.

See scala-native/scala-native#3872

[mpilquist]

scala-native 0.5.1 should be out today, which also unblocks scalacheck.