[gdb/testsuite] Fix gdb.server/server-kill.exp for remote target

[gdb/testsuite] Fix gdb.server/server-kill.exp for remote target

In commit 80dc83fd0e7 ("gdb/remote: handle target dying just before a stepi")
an observation is made that test-case gdb.server/server-kill.exp claims to
kill gdbserver, but actually kills the inferior.  Consequently, the commit
adds testing of killing gdbserver alongside.

The problem is that:
- the original observation is incorrect (possibly caused by misreading getppid
  as getpid)
- consequently, the test-case doesn't test killing the inferior, instead it
  tests killing gdbserver twice
- the method to get the gdbserver PID added in the commit doesn't work
  for target board remote-gdbserver-on-localhost, it returns the
  PID of the ssh client session instead.

Fixing the method for getting the inferior PID gives us fails, and there's no
evidence that killing the inferior ever worked.

So, fix this by reverting the commit and just killing gdbserver, using the
original method of getting the gdbserver PID which does work for target board
remote-gdbserver-on-localhost.

Tested on x86_64-linux.