sad_test: remove unused globals

`comp_pred_` and `comp_pred_test_` have been unused since:
 6228f47c55 Relocate aom_dist_wtd_comp_avg_pred tests

Fixes warnings with `clang version 23.0.0git
(https://chromium.googlesource.com/a/external/github.com/llvm/llvm-project
20b6ec66967ac2a8f932863c1abf251e5b17a843)`:

```
test/sad_test.cc:339:19: warning: variable 'comp_pred_' set but not used [-Wunused-but-set-global]
  339 |   static uint8_t *comp_pred_;
      |                   ^
test/sad_test.cc:342:19: warning: variable 'comp_pred_test_' set but not used [-Wunused-but-set-global]
  342 |   static uint8_t *comp_pred_test_;
      |
```

Bug: 498987224
Change-Id: Ia8ccb21200cce8373b5c6a6be084f33e31b2be71

test/sad_test.cc

diff --git a/test/sad_test.cc b/test/sad_test.cc
index 416f0ee..9800d2d 100644
--- a/test/sad_test.cc
+++ b/test/sad_test.cc
@@ -139,16 +139,12 @@
       source_data_ = source_data8_;
       reference_data_ = reference_data8_;
       second_pred_ = second_pred8_;
-      comp_pred_ = comp_pred8_;
-      comp_pred_test_ = comp_pred8_test_;
     } else {
       use_high_bit_depth_ = true;
       bit_depth_ = static_cast<aom_bit_depth_t>(bd_);
       source_data_ = CONVERT_TO_BYTEPTR(source_data16_);
       reference_data_ = CONVERT_TO_BYTEPTR(reference_data16_);
       second_pred_ = CONVERT_TO_BYTEPTR(second_pred16_);
-      comp_pred_ = CONVERT_TO_BYTEPTR(comp_pred16_);
-      comp_pred_test_ = CONVERT_TO_BYTEPTR(comp_pred16_test_);
     }
     mask_ = (1 << bit_depth_) - 1;
     source_stride_ = (width_ + 31) & ~31;
@@ -336,10 +332,8 @@
   static uint16_t *reference_data16_;
   static uint16_t *second_pred16_;
   int reference_stride_;
-  static uint8_t *comp_pred_;
   static uint8_t *comp_pred8_;
   static uint16_t *comp_pred16_;
-  static uint8_t *comp_pred_test_;
   static uint8_t *comp_pred8_test_;
   static uint16_t *comp_pred16_test_;
   DIST_WTD_COMP_PARAMS jcp_param_;
@@ -529,8 +523,6 @@
 uint8_t *SADTestBase::source_data_ = nullptr;
 uint8_t *SADTestBase::reference_data_ = nullptr;
 uint8_t *SADTestBase::second_pred_ = nullptr;
-uint8_t *SADTestBase::comp_pred_ = nullptr;
-uint8_t *SADTestBase::comp_pred_test_ = nullptr;
 uint8_t *SADTestBase::source_data8_ = nullptr;
 uint8_t *SADTestBase::reference_data8_ = nullptr;
 uint8_t *SADTestBase::second_pred8_ = nullptr;