stub: Remove monitor-fd.c

stub: Remove monitor-fd.c

Both monitor-fd.c and monitor-internal.c contain a stub for
monitor_get_fd(), which causes a duplicate symbol linker error when
linking rust-qemu-api-integration. Use monitor-internal.c instead of
monitor-fd.c and remove the latter.

Reported-by: Zhao Liu <zhao1.liu@intel.com>
Suggested-by: Zhao Liu <zhao1.liu@intel.com>
Fixes: fccb744f41c6 ("gdbstub: Try unlinking the unix socket before binding")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250217104900.230122-1-iii@linux.ibm.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>