Remove rfail support from incremental tests

Incremental revisions beginning with `rfail` would cause the incremental test
runner to build the test program, expecting success, and then run the test
program, expecting failure.

Expecting incremental tests to fail at runtime is of questionable utility,
because in almost all cases an equivalent test program can be made to succeed
at runtime instead.

Removing `rfail` support is a small step towards cleaning up compiletest's
incremental test runner, and its overall handling of pass/fail expectations.

There was one existing regression test using `rfail` revisions:
`tests/incremental/issue-80691-bad-eval-cache.rs`. The test code is complex,
and reverting the fix in RUST-83220 does not actually cause the test to fail,
suggesting that it is no longer a useful regression test. This commit therefore
deletes that test.

Author: Zalathar

src/tests/compiletest.md

@@ -158,9 +158,9 @@ then runs the compiler for each revision, reusing the incremental results from p
 
 The revisions should start with:
 
-* `rpass` — the test should compile and run successfully
-* `rfail` — the test should compile successfully, but the executable should fail to run
 * `cfail` — the test should fail to compile
+* `cpass` — the test should compile successully
+* `rpass` — the test should compile and run successfully
 
 To make the revisions unique, you should add a suffix like `rpass1` and `rpass2`.