testsuite: Remove refactoring transforms that are no-ops

[ahomescu]

The test suite runs a bunch of transforms right now that are no-ops because they only operate on marked nodes (generally with the target mark) and we do not give them any:

• remove_null_terminator (see https://c2rust.com/manual/c2rust-refactor/commands.html#remove_null_terminator)
• bytestr_to_str
• convert_format_args

We could try to add the target mark to all relevant nodes, but for now it's easier to just disable those transforms.