Remove dummy method implementation based on vehicle for AC_Fence

[shiv-tyagi]

We suggest that AC_FENCE_ENABLED can be set to 0, 1 or 2, which is against the pattern followed throughout the codebase. In the AP_*_ENABLED world, something should either be ENABLED or DISABLED, nothing in between.

It is difficult to maintain the current pattern for AC_FENCE and as suggested by @peterbarker in #31716, we should just simply avoid vehicle based dummy method implementation and let blimp/tracker have fences.

[peterbarker]

CI must pass, but I am in favour.

We should work out why Blimp didn't fail CI for the same reason tracker didn't...

[andyp1per]

Love it!

[IamPete1]

This does add the full fence lib to both tracker and blimp

Binary Name      Text [B]         Data [B]      BSS (B)      Total Flash Change [B] (%)      Flash Free After PR (B)
---------------  ---------------  ------------  -----------  ----------------------------  -------------------------
ardurover        0 (0.0000%)      0 (0.0000%)   0 (0.0000%)  0 (0.0000%)                                      491972
arducopter       0 (0.0000%)      0 (0.0000%)   0 (0.0000%)  0 (0.0000%)                                      349564
arduplane        0 (0.0000%)      0 (0.0000%)   0 (0.0000%)  0 (0.0000%)                                      355208
arducopter-heli  0 (0.0000%)      0 (0.0000%)   0 (0.0000%)  0 (0.0000%)                                      352492
antennatracker   9184 (+0.7739%)  4 (+0.1761%)  0 (0.0000%)  9188 (+0.7728%)                                  767968
blimp            9224 (+0.7649%)  4 (+0.1748%)  0 (0.0000%)  9228 (+0.7638%)                                  748604
ardusub          0 (0.0000%)      0 (0.0000%)   0 (0.0000%)  0 (0.0000%)                                      507036

Probably not a massive deal since there not short on flash, but from CI it seems like the fence params are showing up on tracker which they shouldn't since they won't do anything. (As Peter says I would have expected blimp to fail for the same reason)