runtime: remove debugCachedWork

debugCachedWork and all of its dependent fields and code were added to
aid in debugging issue #27993. Now that the source of the problem is
known and mitigated (via the extra work check after STW in gcMarkDone),
these extra checks are no longer required and simply make the code more
difficult to follow.

Remove it all.

Updates #27993

Change-Id: I594beedd5ca61733ba9cc9eaad8f80ea92df1a0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/262350


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>