Remove config options from configure

As of Qt 5, compiler detection is nowadays smart enough to figure out most of the things for us. In particular, -arch is nowadays disregarded, and -no-neon is harmful, as it only serves for disabling neon on supported platforms.

On a side note, we can/should as well remove -platform, and -developer-build is apparently triggered by no variable - on our OBS it is always built as -developer-build.