Remove 'template field' from parsing logic in IndexTemplateMetaData

[martijnvg]

Parsing the template field has been deprecated a long time in IndexTemplateMetaData.Builder.fromXContent(...) method and should be removed.

Originates from: #38448 (comment)

[elasticmachine]

Pinging @elastic/es-core-features

[jakelandis]

This should only be merged to 7.x since we have some internal clients on 6.x that still use the template field

[PnPie]

Hello,
I created a PR for this ;)

[dakrone]

Closing this since the PR that removed it has been merged.