coverage: Use normal edition: headers in coverage tests

Some of these tests were originally written as part of a custom `run-make`
test, so at that time they weren't able to use the normal compiletest header
directive parser.

Now that they're properly integrated, there's no need for them to use
`compile-flags` to specify the edition, since they can use `edition` instead.

tests/coverage/fn_sig_into_try.coverage

@@ -1,5 +1,5 @@
    LL|       |#![feature(coverage_attribute)]
-   LL|       |// compile-flags: --edition=2021
+   LL|       |// edition: 2021
    LL|       |
    LL|       |// Regression test for inconsistent handling of function signature spans that
    LL|       |// are followed by code using the `?` operator.