Misc cosmetic cleanups (#12598)

- Remove a few obsolete "allowed error messages" from tests. The
pageserver doesn't emit those messages anymore.

- Remove misplaced and outdated docstring comment from
`test_tenants.py`. A docstring is supposed to be the first thing in a
function, but we had added some code before it. And it was outdated, as
we haven't supported running without safekeepers for a long time.

- Fix misc typos in comments

- Remove obsolete comment about backwards compatibility with safekeepers
without `TIMELINE_STATUS` API. All safekeepers have it by now.

compute_tools/src/compute.rs

@@ -1286,9 +1286,7 @@ impl ComputeNode {
 
         // In case of error, log and fail the check, but don't crash.
         // We're playing it safe because these errors could be transient
-        // and we don't yet retry. Also being careful here allows us to
-        // be backwards compatible with safekeepers that don't have the
-        // TIMELINE_STATUS API yet.
+        // and we don't yet retry.
         if responses.len() < quorum {
             error!(
                 "failed sync safekeepers check {:?} {:?} {:?}",