[gdb/testsuite] Remove ${board}_file from remote-stdio-gdbserver.exp

Looking at the implementation of ${board}_file in remote-stdio-gdbserver.exp,
I don't see a relevant difference with the implementation of standard_file
in dejagnu.

Simplify the board by removing ${board}_file.

Tested on x86_64-linux, by running gdb.testsuite/board-sanity.exp.