refactor(rustdoc): Remove redundant langstr checks

[epage]

These same checks feed into doctest.can_be_merged, making them redundant.

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[epage]

For quick reference:

rust/src/librustdoc/doctest.rs

Lines 1018 to 1023 in 27050c0

	let is_standalone = !doctest.can_be_merged
	|| scraped_test.langstr.compile_fail
	|| scraped_test.langstr.test_harness
	|| scraped_test.langstr.standalone_crate
	|| self.rustdoc_options.nocapture
	|| self.rustdoc_options.test_args.iter().any(|arg| arg == "--show-output");

rust/src/librustdoc/doctest/make.rs

Lines 120 to 123 in 27050c0

	let can_merge_doctests = can_merge_doctests
	&& lang_str.is_some_and(|lang_str| {
	!lang_str.compile_fail && !lang_str.test_harness && !lang_str.standalone_crate
	});