Revert "Refactor mkFlag / shouldUsePkg into the nixpkgs libraries"

This reverts commit 25a148fa19.

lib/customisation.nix

@@ -158,27 +158,4 @@ rec {
       drv' = (lib.head outputsList).value;
     in lib.deepSeq drv' drv';
 
-
-  /* Tests whether a derivation can be used by the current platform
-     Returns the derivation if true, otherwise null. */
-  shouldUsePkgSystem = system: pkg_: let pkg = (builtins.tryEval pkg_).value;
-    in if lib.any (x: x == system) (pkg.meta.platforms or [])
-      then pkg
-      else null;
-
-  /* Returns a configure flag string in an autotools format
-     trueStr: Prepended when cond is true
-     falseStr: Prepended when cond is false
-     cond: The condition for the prepended string type and value
-     name: The flag name
-     val: The value of the flag only set when cond is true */
-  mkFlag = trueStr: falseStr: cond: name: val:
-    if cond == null then null else
-      "--${if cond != false then trueStr else falseStr}${name}"
-      + "${if val != null && cond != false then "=${val}" else ""}";
-
-  /* Flag setting helpers for autotools like packages */
-  mkEnable = mkFlag "enable-" "disable-";
-  mkWith = mkFlag "with-" "without-";
-  mkOther = mkFlag "" "" true;
 }