Revert "[EmitC][NFC] Introduce common baseclass for conversions and s…

…implify pattern insertion" (iree-org#15265)

Reverts iree-org#14717, since it broke some Windows builds (see
iree-org#14717 (comment),
https://github.com/openxla/iree/actions/runs/6614754015/job/17965409798#step:8:7604)