fsm_extract: avoid calling log_signal to determine wire name

fsm_extract: avoid calling log_signal to determine wire name

log_signal can result in a string with spaces (when bit selection is
involved), which breaks the rule of IdString not containing whitespace.
Instead, remove the sigspec from the name entirely — given that the
resulting wire will have no users, it will be removed later anyway,
so its name doesn't really matter.

Fixes #2118