OPTIM: vars: remove the unneeded lock in vars_prune_*

OPTIM: vars: remove the unneeded lock in vars_prune_*

vars_prune() and vars_prune_all() take the variable lock while purging
all variables from a head. However this is not needed:
  - proc scope variables are only purged during deinit, hence no lock
    is needed ;
  - all other scopes are attached to entities bound to a single thread
    so no lock is needed either.

Removing the lock saves about 0.5% CPU on variables-intensive setups,
but above all simplify the code, so let's do it.