net: qualcomm: rmnet: Remove unused function declaration

rmnet_map_demultiplex() is only declared but not defined anywhere,
so remove it.

CRs-Fixed: 2156182
Change-Id: I732b7028b5d61de7a160b660aceeea0ad95e4763
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>