[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.

gdb/testsuite/gdb.base/vfork-follow-parent.exp

@@ -30,7 +30,9 @@ if { [build_executable "compile $binfile3" $binfile3 $srcfile2] } {
     return -1
 }
 
-set remote_exec_prog [gdb_remote_download target $binfile3]
+if { [is_remote target] } {
+    gdb_remote_download target [standard_output_file $binfile3]
+}
 
 set opts [list debug additional_flags=-DTEST_EXIT]
 if { [build_executable "compile ${binfile}" ${binfile} ${srcfile} ${opts}] } {