Fix a very stale comment.  ActionInput doesn't expose digest (although
it might be handy if it did).
RELNOTES: None.
@@ -17,8 +17,7 @@ package com.google.devtools.build.lib.actions;
import com.google.devtools.build.lib.vfs.PathFragment;
/**
- * Represents an input file to a build action, with an appropriate relative path and digest
- * value.
+ * Represents an input file to a build action, with an appropriate relative path.
*
* <p>Artifact is the only notable implementer of the interface, but the interface remains
* because 1) some Google specific rules ship files that could be Artifacts to remote execution
