Remove warning that interpretation of .drv has changed

This was first tagged as 2.15.0, 1½ years ago; plenty of time for
everyone to catch up.

By now, the warning is causing more confusion than that it is helpful,
because passing a `.drv` or `drvPath` has legitimate use cases.

(cherry picked from commit f534a7a524)

src/libcmd/installable-derived-path.cc

@@ -32,16 +32,6 @@ InstallableDerivedPath InstallableDerivedPath::parse(
         // store path.
         [&](const ExtendedOutputsSpec::Default &) -> DerivedPath {
             auto storePath = store->followLinksToStorePath(prefix);
-            // Remove this prior to stabilizing the new CLI.
-            if (storePath.isDerivation()) {
-                auto oldDerivedPath = DerivedPath::Built {
-                    .drvPath = makeConstantStorePathRef(storePath),
-                    .outputs = OutputsSpec::All { },
-                };
-                warn(
-                    "The interpretation of store paths arguments ending in `.drv` recently changed. If this command is now failing try again with '%s'",
-                    oldDerivedPath.to_string(*store));
-            };
             return DerivedPath::Opaque {
                 .path = std::move(storePath),
             };