[gdb/testsuite] Remove REMOTE_PORTNUM in remote-stdio-gdbserver.exp

[gdb/testsuite] Remove REMOTE_PORTNUM in remote-stdio-gdbserver.exp

The usage for board remote-stdio-gdbserver.exp is advertised as:
...
 # bash$ make check RUNTESTFLAGS="--target_board=remote-stdio-gdbserver \
 #    REMOTE_USERNAME=... REMOTE_HOSTNAME=... REMOTE_PORTNUM=... \
 #    [REMOTE_TMPDIR=${remote_dir}] [GDBSERVER=${remote_gdbserver}]"
...
but when adding REMOTE_PORTNUM=22, I run into:
...
Running stop-reply-no-thread-multi.exp ...
ERROR: tcl error sourcing stop-reply-no-thread-multi.exp.
ERROR: couldn't execute "/usr/bin/ssh -p22": no such file or directory
    while executing
"builtin_spawn {/usr/bin/ssh -p22} -l vries localhost {/usr/bin/gdbserver \
  --once localhost:2346 \
  /home/vries/gdb_versions/devel/build/gdb/testsuite/outp..."
...

Fix this by simply removing REMOTE_PORTNUM.

Tested on x86_64-linux.