ARM: kernel: Get rid of thread_info::used_cp[] array

We keep track of which coprocessor triggered a fault in the used_cp[]
array in thread_info, but this data is never used anywhere. So let's
remove it.

Linus did some digging and found out that the last user of this field
was removed in commit bb1a773d ("kill unused dump_fpu() instances").

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>