Revert "arm64: uaccess: Implement unsafe accessors"

Revert "arm64: uaccess: Implement unsafe accessors"

This reverts commit f8475f987c0da18d12b012f0df4f689f4b83f524.

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>