[kernel] Cleanup ifdefs and unused code in direct floppy driver

[ghaerr]

Removes unused or unneeded ifdefs and outdated code not used in ELKS or TLVC, for understandability and maintainability.
Follow-on from importing direct floppy driver in #1716.

Removes BDEV_SIZE_CHK and most CONFIG_FS_XMS_BUFFER ifdefs.
Removes drive "aliasing" from open code.
Will add fsync_dev/invalidate_inodes to floppy_release in separate push.

Considering whether to remove all floppy formatting code.
Considering whether kernel_ds:tmp_floppy_area could be replaced with DMASEG:0, but unsure if DMASEG used separately in driver.

Tested and works with no functional changes in QEMU.

[ghaerr]

Adds latest direct driver enhancements from @Mellvik's Mellvik/TLVC#24.

BTW @Mellvik, I'm not sure its OK to not call fsync_dev(dev) on the last close to floppy_release, since one can't depend on umount doing it - the device could be unmounted and accessed just by open/close of /dev/df0, in which case the buffers would not be synced.