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

There is no evidence of at91_mci in the code whatsoever. Most of the at91
drivers where removed in 802baf0ba6

PR: 218970
Reported by: reezer@reezer.org

Approved by: imp@
Differential Revision: https://reviews.freebsd.org/D29193

share/man/man4/mmc.4

@@ -23,7 +23,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 8, 2008
+.Dd June 10, 2021
 .Dt MMC 4
 .Os
 .Sh NAME
@@ -46,7 +46,6 @@ bus typically has only one slot, and only memory cards.
 MultiMediaCards exist only in memory.
 SD Cards exist as memory, I/O, or combination cards.
 .Sh SEE ALSO
-.Xr at91_mci 4 ,
 .Xr mmcsd 4 ,
 .Xr sdhci 4
 .Rs

share/man/man4/mmcsd.4

@@ -23,7 +23,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 22, 2008
+.Dd June 10, 2021
 .Dt MMCSD 4
 .Os
 .Sh NAME
@@ -36,7 +36,6 @@ The
 .Nm
 driver implements direct access block device for MMC and SD memory cards.
 .Sh SEE ALSO
-.Xr at91_mci 4 ,
 .Xr mmc 4 ,
 .Xr sdhci 4
 .Rs