bgpd: fix "clear bgp" commands

bgpd: fix "clear bgp" commands

The use of VTY_DECLVAR_CONTEXT(bgp, bgp) here is wrong as we are not
inside the "router bgp" configuration node. This was making the clear
commands always fail with a "Current configuration object was deleted
by another process" error, which doesn't make any sense.

Also, the bgp_clear() function will already check for us if the given
peer-group or neighbor exists or not, there's no need to duplicate this
logic here.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>