mhi: dev: net_dev: remove background worker thread for allocating memory

Netdev driver is adding a reclaim algorithm to recycle memory.
Therefore, background thread is no longer needed since we won't
be running into out of memory situations.

CRs-Fixed: 2361266
Change-Id: Iea52bf963f24a0df1f81d04e6569551971b38f20
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>