Avoid having to unload file in gdb.server/connect-with-no-symbol-file.exp

gdb.server/connect-with-no-symbol-file.exp's connect_no_symbol_file
does:

	gdb_test "file" ".*" "discard symbol table" \
	    {Discard symbol table from `.*'\? \(y or n\) } "y"

A following patch will make gdb_test expect the question out of GDB if
one is passed down as argument to gdb_test.  With that, this test
starts failing.  This is because connect_no_symbol_file is called in a
loop, and the first time around, there's a loaded file, so "file" asks
the "Discard symbol table ... ?" question, while in the following
iterations there's no file, so there's no question.

Fix this by not loading a file into GDB in the first place.

Change-Id: I810c036b57842c4c5b47faf340466b0d446d1abc