gdb/testsuite: fix pattern in gdb.base/dlmopen-ns-ids.exp

I forgot one spot when updating the "info shared" header from NS to
"Linker NS", fix that.  This fixes the following failure:

    FAIL: gdb.base/dlmopen-ns-ids.exp: check no duplicates: info sharedlibrary

At the same time, fix a couple of things I found when looking at this
code again.  One is bad indentation, the other is an unnecessary
parameter.

Change-Id: Ibbc2062699264dde08fd3ff7c503524265c73b0c