ci: reduce number of targets

These targets aren't particular popular.

There aren't any such devices according to map.ffrn.de

Removing them allows for a quicker turnaround while also preventing the waste of CPU cycles.

They can be added again at any time if required.

.github/build-info.json

@@ -13,12 +13,7 @@
             "armsr-armv8",
             "ath79-generic",
             "ath79-nand",
-            "ath79-mikrotik",
-            "bcm27xx-bcm2708",
-            "bcm27xx-bcm2709",
             "ipq40xx-generic",
-            "ipq40xx-mikrotik",
-            "ipq806x-generic",
             "lantiq-xrx200",
             "lantiq-xway",
             "mediatek-filogic",
@@ -28,9 +23,7 @@
             "ramips-mt7620",
             "ramips-mt7621",
             "ramips-mt76x8",
-            "realtek-rtl838x",
             "rockchip-armv8",
-            "sunxi-cortexa7",
             "x86-64",
             "x86-generic",
             "x86-geode"