[gdb/testsuite] Simplify gdb.server/server-kill-python.exp

[gdb/testsuite] Simplify gdb.server/server-kill-python.exp

In test-case gdb.server/server-kill-python.exp we have:
...
if {[gdb_spawn_with_cmdline_opts \
         "-quiet -iex \"set height 0\" -iex \"set width 0\" -ex \"source $host_file1\""] != 0} {
    fail "spawn"
    return
}
...

I reproduced the problem by reverting the fix at the commit adding both the
fix and the test-case, and the reproduced the same problem using:
...
(gdb) source $host_file1
...
so there doesn't seem to be a specific need to source the python file using
"-ex".

Simplify the test-case by sourcing the python file using send_gdb.

This also allow us to simplify the python script.

Tested on x86_64-linux.