[CMake] Remove check for the readline target.
[CMake] Remove check for the readline target.

This was introduced when we were building a custom readline Python
module on Linux [1]. Now that the readline  target doesn't exist
anymore, it's safe to remove this dependency.

This fixes https://llvm.org/PR25136

[1] https://reviews.llvm.org/D13268

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@367956 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb