login: Remove redundant condition.

Treating pointers as unsigned integers, "end - start >= 0" always holds
true.  The documentation of SerializeWithCachedSizesToArray() says
"Returns a pointer past the last written byte" which in my reading
guarantees that |end| is never smaller than |start|.  (A quick glance at
the code seemed to support that interpretation.)

BUG=none
TEST=none

Change-Id: I872a1db7cc87e4fb5658f02678cb8a4a0e5e45b5
Reviewed-on: https://chromium-review.googlesource.com/365830
Commit-Ready: Thiemo Nagel <tnagel@chromium.org>
Tested-by: Thiemo Nagel <tnagel@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>