gdb/testsuite: use clean_restart in gdb.server/solib-list.exp

I think this sequence of commands can be replaced by clean_restart,
despite what the comment says, as long as we don't use the `binfile`
argument to clean_restart.

gdb/testsuite/ChangeLog:

	* gdb.server/solib-list.exp: Use clean_restart.

Change-Id: I4930564c50a1865cbffe0d660a4296c9d2158084