riscv: Remove PGDIR_SIZE_L3 and TASK_SIZE_MIN

TASK_SIZE_MIN is unused since commit 085e2ff9

 ("efi: libstub: Drop
randomization of runtime memory map"). PGDIR_SIZE_L3 is only used in the
definition of TASK_SIZE_MIN.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240327143858.711792-2-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>