Remove unnecessary num_outputs properties

Summary:
These have become are unnecessary in non-multi-output models with D14709315.
Let's remove them to keep things clean.

Reviewed By: danielrjiang

Differential Revision: D14839611

fbshipit-source-id: ebc8f547eab4c3253214253b034c5018d231b350

Author: Balandat

botorch/models/gp_regression.py

@@ -77,10 +77,6 @@ def __init__(
         )
         self.to(train_X)
 
-    @property
-    def num_outputs(self) -> int:
-        return 1
-
     def forward(self, x: Tensor) -> MultivariateNormal:
         mean_x = self.mean_module(x)
         covar_x = self.covar_module(x)