[MLIR] Fix -Wunused-template (#221480) (#223094)

This warning was enabled by default for clang 23 (although reverted in
23.1.1). Given someone might enable it in the future and the cleanup
seemed nice, we should just do it.

This was done primarily by removing static from templates in headers and
then deleting any unused templates in translation units.

Previously attempted to land this in #221480, but accidentally merged
into a user branch rather than main.

GitOrigin-RevId: 4e3b379b6cfcd13a11e26834756519430a9f9377