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/issue-93054.coverage

@@ -1,11 +1,10 @@
    LL|       |#![allow(dead_code, unreachable_code)]
+   LL|       |// edition: 2021
    LL|       |
    LL|       |// Regression test for #93054: Functions using uninhabited types often only have a single,
    LL|       |// unreachable basic block which doesn't get instrumented. This should not cause llvm-cov to fail.
    LL|       |// Since these kinds functions can't be invoked anyway, it's ok to not have coverage data for them.
    LL|       |
-   LL|       |// compile-flags: --edition=2021
-   LL|       |
    LL|       |enum Never {}
    LL|       |
    LL|       |impl Never {