Remove option to ignore negative memberships (#6665)

Remove option to ignore negative memberships (#6665)

Fixes #6661. The option `--strings-inm` could be used to ignore negative
membership constraints. However, this option made the string solver
model-unsound or produced incorrect models if the user provided a
benchmark that actually contained negative membership constraints. The
solver did not check for negative membership constraints and did not
warn the user about them. Because the option is not really being used,
this commit removes it.