dl_dyld.xs should not create a `mode` variable only to ignore it

[nwc10]

This unused variable dates back to when the file was first added. The code was structured as a static C wrapper emulating the dlopen() API with the native APIs, and then a copy of the existing XS code that called dlopen(). However, there is (and was) no need to keep it exactly this way, as the wrapper is only visible inside this file, meaning that it does not need to conform to any externally defined prototype.