refactor(map): remove extra-allocating map_new/map_free functions

Note: the reason for removing them is not that there after this refactor
is no use of them, but rather that having them available is an
anti-pattern: they manange an _extra_ heap allocation which has
nothing to do with the functionality of the map itself (khash
manages the real buffers internally). In case there happens to
be a reason to allocate the map structure itself later, this
should be made explicit using xcalloc/xfree calls.

src/nvim/generators/gen_api_dispatch.lua

@@ -321,8 +321,6 @@ end
 output:write([[
 void msgpack_rpc_init_method_table(void)
 {
-  methods = map_new(String, MsgpackRpcRequestHandler)();
-
 ]])
 
 for i = 1, #functions do