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ARROW-15552: [Doc][Format] Remove erroneous mention of base64

This is probably a leftover of a draft version of the C data interface which involved base64 encoding of metadata values.

Closes #12361 from pitrou/ARROW-15552-c-data-interface-base64

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>