Remove local variable from simple.rs test case

Remove local variable from simple.rs test case

This removes the "y0" variable from simple.rs:main.  This variable
isn't needed by the test case, and it uses a form of initialization
that was added in rust 1.17.  Removing this makes it simpler to run
the gdb.rust tests against older versions of rustc.

gdb/testsuite/ChangeLog
2020-04-01  Tom Tromey  <tromey@adacore.com>

	* gdb.rust/simple.rs (main): Remove "y0".