The `-Znll-subminimal-causes` option is unused

[bjorn3]

https://github.com/rust-lang/rust/search?q=nll_subminimal_causes shows only:

rust/src/librustc/session/config.rs

Lines 1322 to 1323 in 0d4f919

	nll_subminimal_causes: bool = (false, parse_bool, [UNTRACKED],
	"when tracking region error causes, accept subminimal results for faster execution."),

The only usage of the option is in the test dropck_vec_cycle_checked.rs:

rust/src/test/ui/span/dropck_vec_cycle_checked.rs

Line 11 in b7c6e8f

// compile-flags: -Z nll-subminimal-causes