fix: remove delegator addess from MsgCreateValidator

[0xbala-k]

Description

Remove delegator address field from MsgCreateValidator and other related checks, as one cannot create a validator on behalf of other address (i.e. both delegator and validator address are always same while creating a new validator).

Closes: #4595

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[alexanderbez]

Thanks for the contrib @0xmuralik! As others have pointed out, we cannot remove fields from Proto schemas. Rather, you must explicitly mark them as deprecated and then you can remove/modify all associated business logic.

[julienrbrt]

Given that the field is deprecated, we may want to return an error when it is being used?
This makes it state breaking however. But we've done that from bank multisend (going from multiples inputs to only one supported)

[atheeshp]

Maybe we can ignore the value without giving error?

[alexanderbez]

Left one comment on making the docs clearer, but other LGTM!

[tac0turtle]

utACK

[julienrbrt]

lgtm, can you add a changelog entry as well?

[alexanderbez]

Nice! Thanks for the contribution! Once a changelog entry is in, we'll merge 🙌