[gdb/testsuite] Cleanup unnecessary expr from require line

[gdb/testsuite] Cleanup unnecessary expr from require line

In a recent commit I've added:
...
require {expr [have_compile_flag -fsplit-stack]}
...
but actually the expr bit is unnecessary, and we can just use:
...
require {have_compile_flag -fsplit-stack}
...

Reported-By: Tom Tromey <tom@tromey.com>