moby: drop CMA_SIZE_MBYTES kernel override

now that i'm building a size-optimized image, this WORKS

pkgs/additional/linux-sane-pinephonepro/default.nix

@@ -246,7 +246,7 @@ linux_latest.override {
 
         # TODO: at least one of these is required for boot
         # UNINLINE_SPIN_UNLOCK = yes;  #< DOESN'T TAKE EFFECT
-        CMA_SIZE_MBYTES = lib.mkForce (freeform "256");
+        # CMA_SIZE_MBYTES = lib.mkForce (freeform "256");
         SOFTLOCKUP_DETECTOR_INTR_STORM = no;
         LDM_PARTITION = lib.mkForce no;
         DAMON = lib.mkForce no;