QL: remove constant_keyword from code base

[astefan]

Following #58315 merging, there is no need for constant_keyword field type in QL projects anymore.

[elasticmachine]

Pinging @elastic/es-ql (:Query Languages/SQL)

[keremh]

Hello @astefan, i want to work on this issue 👍

[astefan]

Hi @keremh , thank you for your interest in helping out with this gh issue. There is a PR already opened for this which is waiting on bwc tests to be complete and further merged in master afterwards.

[bpintea]

Fixed by #60524.