Don't redefine upload/download/file in gdbserver-base

Don't redefine upload/download/file in gdbserver-base

Before patch

  Make native gdbserver boards no longer be "remote" (in DejaGnu terms)
  739b3f1d8ff7072dcc66240c25b026c6433bda1a

the local gdbserver boards (except native-extended-gdbserver...) were
considered as remote by DejaGNU.  To avoid DejaGNU trying to use ssh/scp
to download the files to the target (which is actually local), the
gdbserver-base.exp file defined some _download, _upload and _file board
operations to override the default behavior, and instead just use local
operations.

The same patch also changed remote-stdio-gdbserver.exp to make it
inherit from gdbserver-base.exp.  Since then, this board (which is
actually remote) uses the overrides with local file operations.  As a
result, files are never actually copied to the target.

I think we can simply remove the overrides from gdbserver-base.exp.
Because all boards should be properly considered local or remote by
DejaGNU, it should by default use the right method for transferring
files.

gdb/testsuite/ChangeLog:

	PR gdb/22841
	* boards/gdbserver-base.exp (${board}_file, ${board}_download,
	${board}_upload): Remove.