Fix py-parameter.exp for remote host

Test gdb.python/py-parameter.exp expects output "$srcdir/$subdir:\$cdir:\$cwd",
but proc gdb_reinitialize_dir doesn't set $srcdir/$subdir in search
directories on remote host because it doesn't exist on remote host.

proc gdb_reinitialize_dir { subdir } {
    global gdb_prompt

    if [is_remote host] {
	return ""
    }

It causes the fail below:

(gdb) python print (gdb.parameter ('directories'))^M
/tmp/gdb:$cdir:$cwd^M
(gdb) FAIL: gdb.python/py-parameter.exp: python print (gdb.parameter ('directories'))

This patch is to fix this fail by not matching $srcdir/$subdir on remote host.

gdb/testsuite:

2014-10-15  Yao Qi  <yao@codesourcery.com>

	* gdb.python/py-parameter.exp: Don't match $srcdir/$subdir on
	remote host.