xfrm: make xfrm modes builtin

after previous changes, xfrm_mode contains no function pointers anymore
and all modules defining such struct contain no code except an init/exit
functions to register the xfrm_mode struct with the xfrm core.

Just place the xfrm modes core and remove the modules,
the run-time xfrm_mode register/unregister functionality is removed.

Before:

    text    data     bss      dec filename
    7523     200    2364    10087 net/xfrm/xfrm_input.o
   40003     628     440    41071 net/xfrm/xfrm_state.o
15730338 6937080 4046908 26714326 vmlinux

    7389     200    2364    9953  net/xfrm/xfrm_input.o
   40574     656     440   41670  net/xfrm/xfrm_state.o
15730084 6937068 4046908 26714060 vmlinux

The xfrm*_mode_{transport,tunnel,beet} modules are gone.

v2: replace CONFIG_INET6_XFRM_MODE_* IS_ENABLED guards with CONFIG_IPV6
    ones rather than removing them.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>