gdb/testsuite: revert recent changes to gdb.dap/scopes.exp

After the previous commit, the changes to gdb.dap/scopes.exp from the
commit:

  commit 63b862be762e1e6e7ce667c6b4a1a3dd79939bf4
  Date:   Fri Mar 29 16:38:50 2019 +0100

      gdb, gdbserver: Add support of Intel shadow stack pointer register.

Are no longer needed, the test will now happily handle the shadow
stack pointer being unavailable.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33345

Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>