cql3: grammar: unify production for bind variables
cql3: grammar: unify production for bind variables

Since 9b49d27a8 ("cql3: expr: Remove shape_type from bind_variable"),
bind variables no longer remember their context (e.g. if they are
in a scalar or vector comparison, or if they are in an IN or
other relation. Exploit that my merging all of the productions that
generate a bind variables (that are now exactly equal) into a single
marker production.

Closes #10624

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla