[gdb/testsuite] Remove quit in gdb.arch/i386-mpx.exp

[gdb/testsuite] Remove quit in gdb.arch/i386-mpx.exp

When running test-case gdb.arch/i386-mpx.exp with target board
native-gdbserver, I run into:
...
(gdb) PASS: gdb.arch/i386-mpx.exp: verify size for bnd0
Remote debugging from host ::1, port 42328^M
quit^M
A debugging session is active.^M
^M
        Inferior 1 [process 19679] will be killed.^M
^M
Quit anyway? (y or n) monitor exit^M
Please answer y or n.^M
A debugging session is active.^M
^M
        Inferior 1 [process 19679] will be killed.^M
^M
Quit anyway? (y or n) WARNING: Timed out waiting for EOF in server after monitor exit
...

The problem is that the test-case sends a quit at the end (without verifying
the result of this in any way):
...
send_gdb "quit\n"
...

Fix this by removing the quit.

Tested on x86_64-linux.