Revert "Make the "word-at-a-time" helper functions more commonly usable"

This reverts commit f68e556e23d1a4176b563bcb25d8baf2c5313f91.
Revert "ARM: use generic strnlen_user and strncpy_from_user functions"

This reverts commit bb00cf8e4748d42e9ac9580c58ec629c9b53dd10.

Conflicts:
	arch/arm/Kconfig
	arch/arm/include/asm/word-at-a-time.h