unix: drop SPAWN_WAIT_EXEC guard in process.c
unix: drop SPAWN_WAIT_EXEC guard in process.c

Code cleanup. This "experimental" feature was always enabled anyway, might as
well remove the define guard.

urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso