Don't attempt to merge non-existant remotes in t5515

Don't attempt to merge non-existant remotes in t5515

This was actually reverted in 756373da by Junio.  We no longer
support merging the right hand side of a fetchspec in a branch's
branch.$name.merge configuration setting as we interpret these
names as being only those published by the remote we are going to
fetch from.

The older shell based implementation of git-fetch did not report an
error when branch.$name.merge was referencing a branch that does
not exist on the remote and we are running `git fetch` for the
current branch.  The new builtin-fetch does notice this failure
and aborts the fetch, thus breaking the tests.

Junio and I kicked it around on #git earlier today and decided that
the best approach here is to error out and tell the user that their
configuration is wrong, as this is likely more user friendly than
silently ignoring the user's request.  Since the new builtin-fetch
is already issuing the error there is no code change required, we
just need to remove the bad configuration from our test.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>