gdb/testsuite: Remove duplicates from gdb.base/ui-redirect.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/ui-redirect.exp ...
    DUPLICATE: gdb.base/ui-redirect.exp: redirect while already logging: set logging redirect off

Fix by moving the first 'set logging redirect off' to the end of the
previous [with_test_prefix] test block. The statement's purpose is to
clean the on flag set in this previous block, so moving it there makes
sense and does not change the sequence of commands in the test file.

Tested on x86_64-linux.