[gdb/testsuite] Fix copy-to-remote in gdb.base/vfork-follow-parent.exp

[gdb/testsuite] Fix copy-to-remote in gdb.base/vfork-follow-parent.exp

When running test-case gdb.base/vfork-follow-parent.exp, I run into:
...
ERROR: tcl error sourcing gdb/testsuite/gdb.base/vfork-follow-parent.exp.
ERROR: error copying "vforked-prog": no such file or directory
    while executing
"file copy -force $fromfile $tofile"
    (procedure "gdb_remote_download" line 29)
    invoked from within
"gdb_remote_download target $binfile3"
...

Fix this by:
- making the copy-to-remote conditional on is_remote target, and
- allowing gdb_remote_download to find $binfile3 by using
  standard_output_file.

Also remove unused variable remote_exec_prog.

Tested on x86_64-linux.