FindLibAStyle: remove confusing default version

On a system without libastyle installed the output was:

• Could NOT find LibAStyle: Found unsuitable version "0", but required is at least "3.1" (found LibAStyle_LIBRARIES-NOTFOUND)

Without this default it will be:

• Could NOT find LibAStyle (missing: LibAStyle_LIBRARIES LibAStyle_INCLUDE_DIR) (Required is at least version "3.1")

While at it move the code for the version autodetection around a bit to simplify
it a bit.