fix(sunxi-6.18): drop r-spi backport now merged in linux-6.18.y stable

The backport patches.backports/31-clk_sunxi-ng_sun55i-a523-r_Add_missing_
r-spi_module_clock.patch (upstream commit fb20ccf70cf6 by Chen-Yu Tsai)
has been absorbed into the linux-6.18.y stable tree. The hunks now
conflict with code that is already present in
drivers/clk/sunxi-ng/ccu-sun55i-a523-r.c, causing:

  Failed to apply patch .../patches.backports/31-clk_sunxi-ng_sun55i-a523-r
                       _Add_missing_r-spi_module_clock.patch
  Summary: 518 total patches; 517 applied; 1 failed_apply

which breaks every kernel-sunxi-current and kernel-sunxi64-current build.

Verified against linux-6.18.y via gregkh/linux mirror: r_spi_parents,
r_spi_clk, and the CLK_R_SPI / CLK_BUS_R_SPI table entries are already
defined upstream. The backport is now redundant; remove it.

Author: igorpecovnik