msm7x27-common: cleanup & reorganize init scripts

* Remove non-existant/useless/duplicate entries from init.msm7x27.rc;
* Configure shared init.msm7x27.rc to load init.${ro.hardware}.usb.rc
  and init.${ro.hardware}.parts.rc (latter can be for device-specific
  services/config).

Change-Id: I804a4d953fa722c6f1596f2bcfdd7b5caff0fd94