Remove duplicated context-propagation documentation in Micrometer docs

[shakuzen]

We have a copy of the context-propagation docs in the Micrometer docs right now here: https://docs.micrometer.io/micrometer/reference/contextpropagation.html

This was made before context-propagation started publishing its own docs, which are now available at: https://docs.micrometer.io/context-propagation/reference/

We do not need the former and it should be removed as it duplicates the canonical context-propagation docs and it is not versioned with context-propagation like the canonical docs are.