riscv: Remove duplicate CLINT_TIMER selections

Since commit f862bbf4 ("riscv: Allow NOMMU kernels to run in
S-mode") in v6.10, CLINT_TIMER is selected by the main RISCV symbol when
RISCV_M_MODE is enabled.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/ce55529a42fa232cacd580e38866c60701f91095.1738764474.git.geert+renesas@glider.be


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>