[gdb/testsuite] Fix gdbserver path in remote-stdio-gdbserver.exp

[gdb/testsuite] Fix gdbserver path in remote-stdio-gdbserver.exp

With test-case gdb.server/unittest.exp and target board remote-stdio-gdbserver
I run into:
...
(gdb) builtin_spawn /usr/bin/ssh -t -l vries localhost /usr/bin/gdbserver \
  --selftest^M
Selftests have been disabled for this build.^M
UNSUPPORTED: gdb.server/unittest.exp: unit tests
...
due to using the system gdbserver /usr/bin/gdbserver rather than the one from
the build.

Fix this by removing the hard-coding of /usr/bin/gdbserver in
remote-stdio-gdbserver, allowing find_gdbserver to do its work, such that we
have instead:
...
(gdb) builtin_spawn /usr/bin/ssh -t -l vries localhost \
  /data/vries/gdb/leap-15-4/build/gdbserver/gdbserver --selftest^M
Running selftest remote_memory_tagging.^M
Ran 1 unit tests, 0 failed^M
Connection to localhost closed.^M^M
PASS: gdb.server/unittest.exp: unit tests
...

Tested on x86_64-linux.