Refactoring: rename NotePlaintext --> SproutNotePlaintext

Also renames:
CNotePlaintextEntry --> CSproutNotePlaintextEntry
CUnspentNotePlaintextEntry --> CUnspentSproutNotePlaintextEntry

src/wallet/gtest/test_wallet.cpp

@@ -155,7 +155,7 @@ TEST(wallet_tests, find_unspent_notes) {
     EXPECT_FALSE(wallet.IsSpent(nullifier));
 
     // We currently have an unspent and unconfirmed note in the wallet (depth of -1)
-    std::vector<CNotePlaintextEntry> entries;
+    std::vector<CSproutNotePlaintextEntry> entries;
     wallet.GetFilteredNotes(entries, "", 0);
     EXPECT_EQ(0, entries.size());
     entries.clear();