Revert "Fix SBDebugger::CreateTargetWithFileAndArch to accept LLDB_ARCH_DEFAULT."

Also revert "Follow on to: f05dc40c"

After these changes, multiple lldb tests are failing. Calls to
CreateTargetWithFileAndArch(None, None) appear to fail after these
changes.

This reverts commit f05dc40c and
1fba2177.