Remove GetSysDirectory spam

[Dentomologist]

Launching and then immediately closing Dolphin results in 51 (out of 81 total) lines of "Common\FileUtil.cpp:916 I[COMMON]: GetSysDirectory: Setting to [Dolphin Sys path]:". Most lines happen during startup and thus only show up in dolphin.log but 10 also appear in the log widget. Starting a game creates more such lines.

Unless I've misunderstood something the Sys path is constant for any given Dolphin session, and so there's no reason to recreate it each time GetSysDirectory is called.

This PR creates and logs the Sys directory path once, and also does some minor refactoring and removes the extraneous colon at the end of the line.

[Dentomologist]

I made your suggested changes.

[AdmiralCurtiss]

Not sure if this happens in practice, but on Android, s_android_sys_directory can be changed at any time, which would affect subsequent calls to GetSysDirectory(). Can we verify this on Android?

Otherwise seems fine to me.

[JosJuice]

It will change exactly once (from an empty string to a non-empty string). The assert that's in the code should catch any cases of GetSysDirectory being called before it changes. But we may want to add an additional assert to SetSysDirectory to ensure it never changes twice.

[Dentomologist]

I've added the SetSysDirectory assert.