Remove redundant invalid runtime.txt integration test

Since the runtime.txt contents are no longer read, and this
test's result/assertions are identical to the test above it.

Author: edmorley

tests/python_version_test.rs

@@ -421,47 +421,6 @@ fn runtime_txt() {
                 each time it builds.
 
 
-                ERROR: failed to build: exit status 1
-            "}
-        );
-    });
-}
-
-#[test]
-#[ignore = "integration test"]
-fn runtime_txt_invalid_version() {
-    let mut config = default_build_config("tests/fixtures/runtime_txt_invalid_version");
-    config.expected_pack_result(PackResult::Failure);
-
-    TestRunner::default().build(config, |context| {
-        assert_contains!(
-            context.pack_stdout,
-            &formatdoc! {"
-                [Determining Python version]
-
-                [Error: The runtime.txt file isn't supported]
-                The runtime.txt file can no longer be used, since it has been
-                replaced by the more widely supported .python-version file.
-                
-                Please delete your runtime.txt file and create a new file named:
-                .python-version
-                
-                Make sure to include the '.' character at the start of the
-                filename. Don't add a file extension such as '.txt'.
-                
-                In the new file, specify your app's major Python version number
-                only. Don't include quotes or a 'python-' prefix.
-                
-                For example, to request the latest version of Python {DEFAULT_PYTHON_VERSION},
-                update your .python-version file so it contains exactly:
-                {DEFAULT_PYTHON_VERSION}
-                
-                We strongly recommend that you don't specify the Python patch
-                version number, since it will pin your app to an exact Python
-                version and so stop your app from receiving security updates
-                each time it builds.
-
-
                 ERROR: failed to build: exit status 1
             "}
         );