fix(sm8550-6.18): drop broken sm8x50 UHS-I/xo-clock mbox patch

The 5-commit mbox series
v2_20260314_vladimir_zapolskiy_arm64_dts_qcom_sm8x50_enable_uhs_i_sdr50_and_sdr104_sd_card_modes.patch
no longer applies against linux-6.18.y:

  Summary: kernel patching: 56 total patches; 51 applied; 5 failed_apply

All five mbox entries fail at sm8450/sm8550/sm8650 .dtsi hunks, blocking
the kernel-sm8550-current build entirely.

The first two commits (sm8550/sm8650 "Fix xo clock supply") were
absorbed upstream — sm8550.dtsi and sm8650.dtsi already use
<&bi_tcxo_div2>, so the -/+ context no longer matches. The remaining
three (sm8450/sm8550/sm8650 "Enable UHS-I SDR50/SDR104") are NOT
upstream yet; dropping them is a temporary regression in SD card UHS-I
modes on those SoCs, accepted to unblock the build.

Follow-up: re-introduce just the three not-yet-upstream commits as a
fresh mbox once someone refreshes them against current 6.18.y context.

Author: igorpecovnik