Remove cross-build configuration from build.sbt.

If sbt-native-packager only supports SBT 0.13+, then scala 2.10
is the required version for the API.

Example errors with "sbt +compile" because SBT 0.13 uses macros:

[error] error while loading package, class file needed by package is missing.
[error] reference value internal of package scala.reflect.macros refers to nonexisting symbol.
[error] error while loading package, class file needed by package is missing.
[error] reference value internal of package scala.reflect.macros refers to nonexisting symbol.
[error] error while loading ProjectExtra, class file needed by ProjectExtra is missing.
[error] reference value internal of package scala.reflect.macros refers to nonexisting symbol.
[error] error while loading ProjectExtra, class file needed by ProjectExtra is missing.
[error] reference value internal of package scala.reflect.macros refers to nonexisting symbol.
[error] error while loading Def, class file needed by Def is missing.