[gdb/testsuite] Use setVariable in gdb.dap/scopes.exp

[gdb/testsuite] Use setVariable in gdb.dap/scopes.exp

The test-case gdb.dap/scopes.exp contains the following outdated comment:
...
 # setVariable isn't implemented yet, so use the register name.
...

Now that setVariable is implemented, use it to set variable scalar, and remove
the bit that sets the first register.  That part is known to fail on s390x,
because the first register isn't writeable [1].

Tested on x86_64-linux.

Suggested-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>
[1] https://sourceware.org/pipermail/gdb-patches/2024-December/213823.html