dwc3: core: remove nominal_elastic_buffer related functionality

On newer platform, GUSB3PIPECTL.ELASTIC_BUFFER_MODE value is set
properly. Hence remove code which modifies this value.

Also remove unused usb3_u1u2_disable variable as u1u2 disable
related functionality is removed now.

Change-Id: Ieac97dc403d0c7223c54d889af39e2b1867d7af1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>