mmc(4), mmcsc(4): Remove missing Xref

[fernape]

Summary

There is no evidence of at91_mci in the code whatsoever. Most of the at91
drivers where removed in 802baf0ba66c18ca52aeaf4a3e7b05e85d8e4d3b

$ grep -R at91 * | grep mci
sys/contrib/device-tree/src/arm/sama5d3_mci2.dtsi:#include
<dt-bindings/pinctrl/at91.h>
sys/contrib/device-tree/src/arm/sama5d3_mci2.dtsi:#include
<dt-bindings/clock/at91.h>

$ man at91_mci
No manual entry for at91_mci

Remove the at91_mci reference from mmc(4) and mmcsc(4) man pages.

PR: 218970
Reported by: [e-mail]

Test Plan

• mandoc -Tlint clean
• igor clean
• aspell happy
• man ./mmc.4 and man ./mmcsc.4 render the page properly

[fernape]

Thanks for the review!