repair/row_level: remove reader timeout

This timeout was added to catch reader related deadlocks. We have not
seen such deadlocks for a long time, but we did see false-timeouts
caused by this, see explanation below. Since the cost now outweight the
benefit, remove the timeout altogether.

The false timeout happens during mixed-shard repair. The
`reader_permit::set_timeout()` call is called on the top-level permit
which repair has a handle on. In the case of the mixed-shard repair,
this belongs to the multishard reader. Calling set_timeout() on the
multishard reader has no effect on the actual shard readers, except in
one case: when the shard reader is created, it inherits the multishard
reader's current timeout. As the shard reader can be alive for a long
time, this timeout is not refreshed and ultimately causes a timeout and
fails the repair.

Refs: scylladb#18269

repair/row_level.cc

@@ -436,11 +436,6 @@ repair_reader::repair_reader(
 future<mutation_fragment_opt>
 repair_reader::read_mutation_fragment() {
     ++_reads_issued;
-    // Use a very long timeout for the reader to break out any eventual
-    // deadlock within the reader. Thirty minutes should be more than
-    // enough to read a single mutation fragment.
-    auto timeout = db::timeout_clock::now() + std::chrono::minutes(30);
-    _reader->set_timeout(timeout);   // reset to db::no_timeout in pause()
     return _reader.value()().then_wrapped([this] (future<mutation_fragment_opt> f) {
         try {
             auto mfopt = f.get();
@@ -486,7 +481,6 @@ void repair_reader::check_current_dk() {
 }
 
 void repair_reader::pause() {
-    _reader->set_timeout(db::no_timeout);
     if (_reader_handle) {
         _reader_handle->pause();
     }