chore: Remove unity builds (#6300)

Unity builds were intended to speed up builds, by bundling multiple files into compilation units. However, now that ccache is available on all platforms, there is no need for unity builds anymore, as ccache stores compiled individual build objects for reuse. This change therefore removes the ability to make unity builds.

.github/scripts/strategy-matrix/linux.json

@@ -208,5 +208,5 @@
     }
   ],
   "build_type": ["Debug", "Release"],
-  "cmake_args": ["-Dunity=OFF", "-Dunity=ON"]
+  "cmake_args": [""]
 }