Disable autoupdate in explorer tests. (#4023)

Just replacing AUTOUPDATE with NOAUTOUPDATE, and removing the autoupdate
script. Tests will still run, but autoupdate may need to be fixed if
significant changes are made.

I noticed this while trying to autoupdate for #4007 (because we verify
that tests have been autoupdated). Autoupdate was likely broken by
#3449.

Trying to run with no changes gives:

```
CHECK failure at testing/file_test/file_test_base.cpp:801: !absl::GetFlag(FLAGS_test_targets_file).empty(): Missing --test_targets_file.
```

This is because the `file_test` rule creates a file with test inputs
that it runs with, which the prebuilt binary doesn't provide.

A local kludge to create a `file_test` target not using prebuilt_binary
showed another error:

```
: CommandLine Error: Option ': CommandLine Error: Option 'parser_debug' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options
trace_phase' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options
```

I'm thinking here that the explorer code hasn't been in enough use, and
we're seeing some rot as a consequence. Rather than trying to maintain
it, I'm suggesting to go with NOAUTOUPDATE.

explorer/testdata/trace/phase_source_program.carbon

@@ -13,7 +13,7 @@ fn Main() -> i32 {
 // Place checks after code so that line numbers are stable, reducing merge
 // conflicts.
 // ARGS: --trace_file=- --trace_phase=source_program %s
-// AUTOUPDATE
+// NOAUTOUPDATE
 
 // CHECK:STDOUT: * * * * * * * * * *  source program  * * * * * * * * * *
 // CHECK:STDOUT: --------------------------------------------------------