mediatek: remove redundant patch

mediatek: remove redundant patch

The patch 921-mt7986-add-mmc-support.patch introduced by commit
dabcaac443 ("mediatek: add mt7986 soc support to the target") has never
been applied in a way that it would have any effect as it actually
created a file target/linux/generic/patches-5.15/... in the kernel tree
and was probably a patch intended to be applied to openwrt.git instead
of being put into kernel patches folder as a file.

As an upstream commit from vanilla Linux also adding support for MT7986
to the mtk-sd driver has already been included we can remove that old
patch.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>