java_sdk_library: Simplify SdkLibrary.sdkJars

The expression !sdkVersion.specified() is equivalent to the following:
    !sdkVersion.valid() || sdkVersion.kind == sdkPrivate

That means that the else if clause "sdkVersion.kind == sdkPrivate" will
never be reached so it was removed.

There is also no need to check sdkVersion.valid() as that will already
have been checked, and an error reported, before this code has been
called. Therefore that has been removed.

Bug: 148080325
Test: m nothing
Change-Id: Ib7c544b48dfb53bad3e15f9f73f71bd05cff9b49