libmd: stop exporting Transform() symbols

[kevans]

Summary

They're not documented in libmd and we don't have any consumers. It's
problematic to keep them exported, as we don't currently export their
implementations. Make them all private.

[delphij]

For libmd, please also remove the _Transform functions from the header (.h) files.

[fuz]

LGTM and exp-ran fine in PR 280784.