arm-trusted-firmware-mediatek: remove DDR combo for MT7987

The DRAM_USE_COMB option is only valid for the MT7988. There
is no DRAM type selection for the MT7987, so remove it.

Fixes: de8fc8b ("arm-trusted-firmware-mediatek: add builds for MT7987")
Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Link: https://github.com/openwrt/openwrt/pull/21000
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit bb2d9c1a65)

package/boot/arm-trusted-firmware-mediatek/Makefile

@@ -408,7 +408,6 @@ define Trusted-Firmware-A/mt7987-emmc-comb
   BOOT_DEVICE:=emmc
   BUILD_SUBTARGET:=filogic
   PLAT:=mt7987
-  DRAM_USE_COMB:=1
 endef
 
 define Trusted-Firmware-A/mt7987-emmc-ddr4-4bg
@@ -424,7 +423,6 @@ define Trusted-Firmware-A/mt7987-nor-comb
   BOOT_DEVICE:=nor
   BUILD_SUBTARGET:=filogic
   PLAT:=mt7987
-  DRAM_USE_COMB:=1
 endef
 
 define Trusted-Firmware-A/mt7987-sdmmc-comb
@@ -432,7 +430,6 @@ define Trusted-Firmware-A/mt7987-sdmmc-comb
   BOOT_DEVICE:=sdmmc
   BUILD_SUBTARGET:=filogic
   PLAT:=mt7987
-  DRAM_USE_COMB:=1
 endef
 
 define Trusted-Firmware-A/mt7987-sdmmc-ddr4-4bg
@@ -448,7 +445,6 @@ define Trusted-Firmware-A/mt7987-spim-nand0-ubi-comb
   BOOT_DEVICE:=spim-nand
   BUILD_SUBTARGET:=filogic
   PLAT:=mt7987
-  DRAM_USE_COMB:=1
   USE_UBI:=1
   SPIM_CTRL:=0
 endef
@@ -458,7 +454,6 @@ define Trusted-Firmware-A/mt7987-spim-nand2-ubi-comb
   BOOT_DEVICE:=spim-nand
   BUILD_SUBTARGET:=filogic
   PLAT:=mt7987
-  DRAM_USE_COMB:=1
   USE_UBI:=1
   SPIM_CTRL:=2
 endef
@@ -468,7 +463,6 @@ define Trusted-Firmware-A/mt7987-ram-comb
   BOOT_DEVICE:=ram
   BUILD_SUBTARGET:=filogic
   PLAT:=mt7987
-  DRAM_USE_COMB:=1
   RAM_BOOT_UART_DL:=1
   HIDDEN:=
   DEFAULT:=TARGET_mediatek_filogic