os: rm reiserfs exception from TestSeek

This exception was originally added by CL 152108 in November 2009. I'm
pretty sure no one uses reiserfs nowadays (and if someone does, this bug
must have been fixed by now).

Change-Id: I274ebe2f7910cab81b70a44b2da3f6fe761073bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/591417


Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>