Simplify ProgramShape API by removing ShortDebugString() and DebugString().

These methods are unused, untested, and unnecessary. Remove them to reduce the API surface. In case their functionality is needed, we can call `.ToProto().DebugString()`, etc, instead, which is clearer (it's obvious what the string format is) and allows the caller to cache the proto object for better performance if needed.

PiperOrigin-RevId: 754016229

Author: Google-ML-Automation

xla/shape.h

@@ -677,9 +677,6 @@ class ProgramShape {
     return parameter_names_;
   }
 
-  std::string ShortDebugString() const { return ToProto().ShortDebugString(); }
-  std::string DebugString() const { return ToProto().DebugString(); }
-
  private:
   // Invariant: parameters_ and parameter_names_ have the same size.