[gdb/testsuite] Unset DEBUGINFOD_URLS on remote host

When running test-case gdb.arch/i386-pkru.exp with host board
local-remote-host-notty and target board native-gdbserver on openSUSE
Tumbleweed (with DEBUGINFOD_URLS set), I run into:
...
This GDB supports auto-downloading debuginfo from the following URLs:^M
  <https://debuginfod.opensuse.org/>^M
Enable debuginfod for this session? (y or [n]) ^CQuit^M
(gdb) FAIL: gdb.arch/i386-pkru.exp: runto: run to main
...

The problem is that the unsetenv for DEBUGINFOD_URLS in default_gdb_init:
...
    # If DEBUGINFOD_URLS is set, gdb will try to download sources and
    # debug info for f.i. system libraries.  Prevent this.
    unset -nocomplain ::env(DEBUGINFOD_URLS)
...
doesn't work on remote host.

Fix this by using "set debuginfod enabled off" for remote host.

Tested on x86_64-linux.