s390/bitops: remove outdated comment

 * end up numbered:

 *   |63..............0|127............64|191...........128|255...........192|

 *

 * There are a few little-endian macros used mostly for filesystem

 * bitmaps, these work on similar bit array layouts, but byte-oriented:

 *   |7...0|15...8|23...16|31...24|39...32|47...40|55...48|63...56|

 *

 * The main difference is that bit 3-5 in the bit number field needs to be

 * reversed compared to the big-endian bit fields. This can be achieved by

 * XOR with 0x38.

 *

 * We also have special functions which work with an MSB0 encoding.

 * The bits are numbered:

 *   |0..............63|64............127|128...........191|192...........255|