libsnapshot: Don't map "b" partitions during testing.

This is testing behavior that no longer exists. We never perform an OTA
while system_other is mapped, since it would collide with the snapshot
for system.

Bug: 208944665
Test: vts_libsnapshot_test
Change-Id: I1e462fad3fd59b8b881d98da982a7c22408b837b
diff --git a/fs_mgr/libsnapshot/snapshot_test.cpp b/fs_mgr/libsnapshot/snapshot_test.cpp
index c3b2f78..7001b9a 100644
--- a/fs_mgr/libsnapshot/snapshot_test.cpp
+++ b/fs_mgr/libsnapshot/snapshot_test.cpp
@@ -897,9 +897,9 @@
         ASSERT_NE(nullptr, metadata);
         ASSERT_TRUE(UpdatePartitionTable(*opener_, "super", *metadata.get(), 0));
 
-        // Map source partitions. Additionally, map sys_b to simulate system_other after flashing.
+        // Map source partitions.
         std::string path;
-        for (const auto& name : {"sys_a", "vnd_a", "prd_a", "sys_b"}) {
+        for (const auto& name : {"sys_a", "vnd_a", "prd_a"}) {
             ASSERT_TRUE(CreateLogicalPartition(
                     CreateLogicalPartitionParams{
                             .block_device = fake_super,