pw_kvs: Reduce stack usage of FlashPartition

Reduce stack usage of FlashPartition::IsRegionErased() by removing the
erased_patter_buffer[]. This is a 256 byte savings of stack usage for
the current Pigweed config.

Change-Id: I018554cdf5011f83977c7d7ccb4dbb1bd4da5109
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/13680
Commit-Queue: David Rogers <davidrogers@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_kvs/flash_partition_test.cc b/pw_kvs/flash_partition_test.cc
index 5e90ac9..df3d1c6 100644
--- a/pw_kvs/flash_partition_test.cc
+++ b/pw_kvs/flash_partition_test.cc
@@ -138,9 +138,17 @@
     ASSERT_EQ(block_size, status.size());
   }
 
+  // Preset the flag to make sure the check actually sets it.
+  bool is_erased = true;
+  ASSERT_EQ(Status::OK,
+            test_partition.IsRegionErased(
+                0, test_partition.size_bytes(), &is_erased));
+  ASSERT_EQ(false, is_erased);
+
   ASSERT_EQ(Status::OK, test_partition.Erase());
 
-  bool is_erased;
+  // Preset the flag to make sure the check actually sets it.
+  is_erased = false;
   ASSERT_EQ(Status::OK,
             test_partition.IsRegionErased(
                 0, test_partition.size_bytes(), &is_erased));