Revert "arm64: uaccess: Implement unsafe accessors"

This reverts commit 0bd3ef34

.

There is ongoing work on objtool to identify incorrect uses of
user_access_{begin,end}. Until this is sorted, do not enable the
functionality on arm64. Also, on ARMv8.2 CPUs with hardware PAN and UAO
support, there is no obvious performance benefit to the unsafe user
accessors.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>