Revert "exfat: always build"

This reverts commit f79827881a6fae5d704642d310d59a10a059a2b4.
Revert "drivers/Makefile: add exFat support"

This reverts commit 804acbc22d91dbebbc4f58c92dbfafeab068ba28.
Revert "drivers/exfat: add Samsung's open source exFat module version 1.2.4"

This reverts commit 781011f84d51bd466457289652dc86bdcb50d7ac.