[libc][NFCI] Remove lane size template argument on RPC server (#84557)

Summary:
We previously changed the data layout for the RPC buffer to make it lane
size agnostic. I put off changing the size for the server case to make
the patch smaller. This patch simply reorganizes code by making the lane
size an argument to the port rather than a templtae size. Heavily
simplifies a lot of code, no more `std::variant`.