[libj6] Remove driver_main

The `driver_main` sinature was an alternate signature for `main`
implemented with weak symbols, but it causes linking issues when not
statically linked, and drivers are going to work differently soon
anyway. Just get rid of it for now.

src/libraries/j6/j6.module

@@ -7,7 +7,6 @@ j6 = module("j6",
         "channel.cpp",
         "condition.cpp",
         "init.cpp",
-        "init.s",
         "memutils.cpp",
         "mutex.cpp",
         "protocol_ids.cpp",