riscv: head: remove useless __PAGE_ALIGNED_BSS and .balign

After commit 83e7b876 ("RISC-V: move empty_zero_page definition to
C and export it"), the empty_zero_page has been moved outside head.S,
the __PAGE_ALIGNED_BSS and .balign LoCs are useless, clean up them.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>