mhi: dev: net_dev: remove processing fragmented buffers

MHI net driver would never receive overflow packets since driver
always allocates buffers large enough to accommodate biggest buffer
size hardware can support. So, removing dead code to simplify the
driver.

CRs-Fixed: 2361266
Change-Id: I687ce03f6d56abca0c2087b4fe9f09d34fc4811d
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>