net: qualcomm: rmnet: Remove redundant check when stamping map header

We already check the headroom once in rmnet_map_egress_handler(),
so this is not needed.

CRs-Fixed: 2156182
Change-Id: I7d9966120a0acfa0fb60a24245ff55be6d5bba2c
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>