Fix Command feature: use get_mods() instead of keyboard_report->mods (#4955)

Fix Command feature: use get_mods() instead of keyboard_report->mods (#4955)

* Remove unnecessary IS_COMMAND definition in clueboard/66/rev4

* Replace keyboard_report->mods with get_mods() in IS_COMMAND definitions

* Update docs to use get_mods() instead of keyboard_report->mods

* Replace keyboard_report->mods with get_mods() in tmk_core/protocol/usb_hid/test

For some reason, this occurrence used KB_LSHIFT and KB_RSHIFT, which don't exist

* Replace keyboard_report->mods with get_mods() in Massdrop keyboards (as per @patrickmt)