batman-adv: refactor tt_global_del_struct()

batadv_tt_global_del_struct() function is not properly named.
Having a more meaningful name which reflects the current behavior helps other
developers to easily understand what it does.

A parameter has also been renamed in order to let the function header better fit
the 80-chars line-width

Signed-off-by: Antonio Quartulli <ordex@autistici.org>