Remove redundant action triggers on persist.sys.usb.config

persist.sys.usb.config stores the usb functions which need to be enabled
when the phone boots up. When the phone is actually booted, setting this
would also activate the following action trigger on sys.usb.config

From init.usb.rc:
Used to set USB configuration at boot and to switch the configuration
when changing the default configuration
on property:persist.sys.usb.config=*
    setprop sys.usb.config ${persist.sys.usb.config}

sys.usb.config is anyways set to the actual functions by UsbDeviceManager
once it is set to none. Therefore add "on boot" condition to the above
action trigger so that persist.sys.usb.config does not race with
sys.usb.config.

BUG: 30440213
Change-Id: I3333d0b8334fb627469c7faad250bf3151bb1ebf